How a primary feed works
A primary feed is the list Merchant Center treats as your catalogue. If a product is not in it, that product does not exist as far as Google is concerned, no matter what your website shows. This is the crucial difference from a supplemental feed, which can only change products that are already there.
You can supply it in several ways: a file Google fetches from a URL on a schedule, a manual upload, a connected Google Sheet, the Content API, or a plugin on your shop platform that pushes the data for you. Most shops running WooCommerce, Shopify or Magento use the platform’s own integration, which is usually the least fragile option because it reads the same database the shop sells from.
Each primary feed is set up for a specific country of sale and language. Selling into two markets means two configurations, and items only become eligible in the markets their feed covers.
Why the primary feed matters
Everything downstream depends on it. Shopping campaigns, the shopping half of Performance Max and free listings all draw from the same catalogue, so a gap in the primary feed is a gap in every one of them at once.
Freshness matters as much as completeness. Prices and stock levels in the feed have to agree with the website, because a mismatch is a disapproval rather than a minor inconsistency, and items stop being eligible if the feed is not refreshed on its schedule. A feed left to go stale takes the whole channel down quietly.
Common mistakes with primary feeds
The most damaging is running more than one primary feed containing the same products. They compete over the same identifiers, the data that wins is not always the data you intended, and nobody notices because both feeds report as healthy.
The second is maintaining a spreadsheet by hand. It works for the first week and then drifts, because the shop keeps trading and the sheet does not. If products are added or repriced regularly, the feed has to come from the shop itself.
Two quieter ones. Accepting a plugin’s default output without reading it leaves you with titles built from internal product names that no customer would ever search for. And a fetch schedule set less often than your stock actually moves guarantees a steady trickle of price and availability mismatches. Corrections belong in a supplemental feed or in feed rules, not in a second primary feed.
How to set one up well
Choose a single source and stay with it: the shop platform where possible, a scheduled file where not. Set the country and language deliberately for each market you sell to, and check that the products you expect actually appear in the account afterwards rather than assuming they did.
Match the fetch schedule to how fast your catalogue changes, and put someone in charge of the diagnostics tab so problems are found before the campaign reports them as poor performance. From there, improvements to titles, categories and attributes are the ordinary work of feed management, layered on top of a primary feed that is doing its job.