In this Merchant Center Mastery video, I walk you through some basics of why and when to use supplemental data sources, and then how to set them up. Look at this less like a simplistic step by step process, and more a masterclass for learning more about supplemental sources!

0:00 Introduction
0:25 How to Enable Supplemental Sources & Attribute Rules in your GMC Next Account
1:10 Brief Overview of Data Sources in Google Merchant Center Next
2:15 When to use a Supplemental Source in Merchant Center Next
5:05 What Data to include in your Supplemental Source?
5:40 Step by Step Walkthrough: How to Add a Supplemental Source
7:41 Create a supplemental source with Google Sheets
11:40 When to use Attribute Rules or Feed Rules instead of a Supplemental Source
13:09 Using a supplemental source for Custom Labels
15:00 Using a supplemental source for Promotion IDs
16:20 Pushing the Supplemental Data Source live
19:25 How to Disconnect a Supplemental Source from a Primary Data Source
21:26 In Conclusion

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Step By Step Walk Through of Adding a Supplemental Data Source in Google Merchant Center Next

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Step 1: Ensure you have enabled the supplemental sources by turning on the Advanced Data Management add-on in Merchant Center Next. See the step by step walkthrough here: Here's How to Enable Supplemental Feeds and Feed Rules in Google Merchant Center Next

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Step 2: Navigate to, and then "Add a supplemental source"

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Step 3: Set the pull time to an hour after your existing feed loads (if applicable), then select the Google Sheets and "Use template"

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Step 4: Sync your Google Account, and then remove all data from the spreadsheet (or, keep the columns you will be using)

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Step 5: Add the product IDs and the attributes you want to bulk edit in the supplemental data source: 

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Step 6: Go back to GMC Next UI view, select "Continue", then select the primary Data Source you want to connect the Supplemental Source to, and select "Create data source"

Step 7: Make sure to manually "Update" the supplemental feed the first time (otherwise it won't run until he first time you have it set to automatically run), and congratulaions! Your supplemental source is now editing your primary source.

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Step 8: Consider changing the name of the supplemental source so you know what this supplemental feed is editing (my recommendation is to keep your supplemental feeds organized so you can easily see what is influencing which data without having to open ever supplemental feed and dig through it).

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How to Set Up Supplemental Data Sources in Google Merchant Center Next (And Why Most People Are Using Them Wrong)

There's a particular kind of feed management mistake I've seen more times than I'd like to admit, and it tends to happen to people who are smart enough to know that supplemental sources exist but haven't quite gotten to the bottom of how they actually work. They pull in their entire product catalog, dump it into a supplemental source, change a few things, and then wonder why their data is behaving strangely six months later. I want to talk about that, and I also want to walk you through how to actually set these up correctly in Merchant Center Next, because the path there is a little less intuitive than it probably should be.

But first, a quick housekeeping note for anyone who has gone looking for supplemental sources and come up empty: they are not on by default in Merchant Center Next, which is a design decision I genuinely cannot explain. To turn them on, you need to go to Add-ons, find the Discover tab, and activate something called Advanced Data Source Management. Once you do that, supplemental sources, feed rules, and attribute rules will all become available to you. If you've been staring at your data sources wondering where everything went, that's why, and now you know.

Change Things at the Source Whenever You Can

Before getting into the mechanics of supplemental sources, I think it's worth spending a moment on a principle that I come back to constantly when I'm managing product data, which is that your first instinct should almost always be to make changes in your primary data source rather than reaching for a supplemental source to patch things over.

The reason this matters more than it might seem is that feed data has a way of accumulating layers over time. You've got your content API pushing data, a supplemental source overriding some of it, maybe a feed rule doing something else on top of that, Google's automated improvements adding yet another layer, and somewhere in there someone has been editing individual products manually. When something goes wrong in that environment (and something always goes wrong eventually), figuring out which layer is causing the problem can become genuinely miserable. Clean data, managed as close to the source as possible, is what keeps that from becoming your Tuesday afternoon.

That said, there are real and legitimate reasons to use supplemental sources, and I don't want to overstate the caution to the point where it sounds like I'm discouraging you from using them at all, because they're actually one of the more powerful tools available to anyone doing serious feed work. The point is just to reach for them when they're the right tool, not the convenient one.

When Supplemental Sources Actually Make Sense

The situations where I find supplemental sources genuinely earn their place tend to cluster around a few scenarios. The first is when you don't have easy access to the primary feed source, which happens more often than you might think. If making a change to your core feed means filing a ticket with a developer and waiting two weeks, and you need to update something now, a supplemental source gives you a path forward that doesn't require anyone's sprint planning.

The second scenario is emergency price corrections. We've used supplemental sources to hard-code correct pricing when a feed was pulling wrong numbers and a suspension was either imminent or already happening. You can push the corrected prices through the supplemental source, get Google to verify the fix, lift the suspension, and then use the breathing room to go back and actually solve the underlying problem in the primary feed. It's a little like using duct tape to stop a leak while you wait for the plumber, which sounds inelegant but is sometimes exactly the right call.

The third scenario, and this is probably the one I find most useful on an ongoing basis, is custom labels. If you want to segment products for campaign targeting in Google Ads, whether for a Black Friday promotion, a clearance push, or just organizing your catalog by price tier or margin, supplemental sources give you a flexible way to manage those custom labels without touching your primary feed every time something changes. You can maintain the campaign structure in Google Ads, keep it pointing at the right custom label, and then simply update which product IDs carry that label in your supplemental source. The campaign keeps working. You just swap the products in and out.

For more on how custom labels connect to campaign structure and bidding decisions, our writing on Google Shopping strategy gets into a lot of this in more depth, and I think it's worth reading alongside this post if feed segmentation is something you're actively working on.

The Mechanics of Setting One Up

When you go to add a supplemental source in Merchant Center Next, you'll have a few options for how to connect it, including API connections and file uploads, but the approach I default to for most use cases is a Google Sheet. It's visible, it's editable, it's easy to share with a client or a colleague, and it keeps everything in one place where you can actually see what's happening.

Google provides a template when you create a new supplemental source via spreadsheet, and my strong preference is to delete the sample content and start clean rather than building on top of the template structure. This is probably just my personality showing, but I think clean data is worth the extra thirty seconds it takes to clear a template, because clarity about what's in your supplemental source is what saves you from confusion later.

The two columns you need in almost every supplemental source are the ID column (matching your product IDs exactly as they appear in your primary feed, which matters more than you'd think since this matching is case sensitive, a detail that has caused unnecessary confusion for a lot of people) and then whatever attribute you actually want to change. That's it. The discipline here is keeping the supplemental source narrow. The only thing that belongs in it is the attribute you're actually trying to modify. Bringing in other attributes "just in case" or as a reference is how supplemental sources quietly start overriding data you didn't mean to touch.

There's a particular mistake worth calling out specifically, which is downloading your entire product catalog and importing it wholesale into a supplemental source before making a few targeted changes. Anything you include in that supplemental source will be hard-coded over whatever your primary feed sends, so if you import five hundred attributes and only mean to change three of them, the other four hundred and ninety-seven are now locked in place and will override any future updates from your primary source. I've seen this cause real problems for people, and it's the kind of thing that's easy to miss until something downstream stops behaving the way you expect.

Once you've set up your source and linked it to a primary data source, you'll need to push an update and let Google process it. You'll then see a match report telling you how many of your product IDs connected successfully. If you're seeing zero matches, the case sensitivity issue is the first thing to check, particularly if you're working with Shopify and copying product IDs from Google Ads reporting, where they sometimes paste in lowercase when they need to be uppercase.

One More Thing Worth Knowing About Disassociating Sources

Rather than deleting a supplemental source when you're done with it, I'd recommend simply disassociating it from your primary data source instead. You do this through the feed rules of the primary source, not from within the supplemental source itself, which is a little counterintuitive. Once disassociated, the supplemental source stops influencing your data but remains available if you need to reattach it later.

This comes in particularly handy for emergency situations. If you've ever had to push a bulk correction through a supplemental source to resolve a suspension, being able to reactivate that source quickly rather than rebuilding it from scratch is the kind of thing you appreciate enormously at ten o'clock on a Tuesday night when a client is messaging you about disapprovals.

For the broader context on how to approach Google Merchant Center diagnostics and feed health, and on the Google policy suspensions side of things where supplemental sources can genuinely save you, there's quite a bit more on the blog that I think fills in the gaps this post leaves open.

The summary I keep coming back to is this: supplemental sources are a genuinely useful tool when they're used with intention and kept as narrow as the task requires. The problems tend to come not from using them, but from using them carelessly, and the difference between the two is mostly just knowing what's actually in there and why. Which, it seems to me, is how most things in feed management tend to go.

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