What is a supplemental data source in Merchant Center?
WeAdU · Updated 5 October 2026
A supplemental data source updates product attributes that are already in a primary data source. It supplies or updates attributes missing from the original file, and it does not add or remove products.
What does this source update?
The help read on 4 October 2026 describes a secondary source. It supplies an attribute missing from the primary source, or it updates an attribute that is already there. Only the primary source adds or removes products. A supplemental source cannot stand on its own. The tab appears only once you turn on the Advanced data source management add-on, under Settings, then Add-ons.
On a multi-client account, these sources can apply to every sub-account or to some of them. If you override the rules and supplemental data of the sub-accounts, you can no longer link a source added at sub-account level to a primary source.
How do the two files join?
The usual join uses the same id, exactly the id on the primary source. The data source label must match the primary source's feed label exactly, or the link fails. The language must match too. That label allows only uppercase letters, digits, a hyphen or an underscore. You then choose the primary source that receives the data.
On a multi-brand catalogue, the original file already holds the products. The supplemental source brings in a separate attribute, for example a custom label, a price or availability. The list is not exhaustive.
What does a join on another attribute allow?
Another page, read the same day, joins the files on an attribute other than the id. The example uses brand, and the page also names product type. Products of one brand then receive a custom label: one row per brand is enough, instead of one row per id.
That page sets a row limit, given on the help page. This join is open only to some merchants, by a support request. The help does not describe that limit for the id join. An id column under another name points to an id rule.
What happened to the online inventory update?
The old online inventory sources, which updated price and availability without reuploading the main file, are deprecated. You get the same function with a supplemental source that holds the id, the price and the availability, linked to the primary source by the Take latest rule, which applies only to price and availability.
Regional inventory is a supplemental source of the same kind. It overrides price or availability in regions that are already defined. It requires the product id and the region id. The optional attributes named are price, sale price, the sale price effective date and availability.
What does the WeAdU audit cover?
The catalogue question is the feed for a multi-brand reseller, including Performance Max. The article linked below describes the campaigns. This page describes the attribute file. All our audits cover Google Ads only. This page does not promise that a product will show, and it does not describe a fix.
Help pages
Google help pages read on 4 October 2026.