What availability states
Availability is the field in a product feed that says whether a shopper can buy the item right now. It takes a small set of fixed values: in stock, out of stock, preorder for something not yet released, and backorder for something you sell but cannot ship immediately. Both Google Merchant Center and a Meta catalogue use the same idea, with minor differences in wording.
The value is not taken on trust. Google reads your landing page and compares what it finds there with what the feed claims. If the page shows a sold-out message while the feed says in stock, that is a mismatch, and the item can be rejected. The check is one of the reasons this field causes more disruption than its simplicity suggests.
Why availability matters
The obvious reason is that advertising something you cannot sell wastes money and irritates people. The less obvious one is that repeated mismatches damage the standing of the whole account, and a broad sweep of disapprovals can take products out of the auction well beyond the ones that actually sold out.
Timing is the crux. Ads keep running on the data Google last fetched, so a product that sells out shortly after an upload continues to be promoted until the next refresh. On a small catalogue with a fast-selling line, that gap is where the money leaks. It also shapes what happens after the click, and a shopper who lands on an out-of-stock page is unlikely to try you again soon.
Where availability goes wrong
Slow updates cause most of the damage. A feed fetched on a fixed daily schedule will always lag real stock, and the busier the trading day the wider the gap grows.
Then there is the temptation to set everything to in stock permanently, because disapprovals stop appearing that way. They stop because the mismatch has moved to the landing page, where it does more harm. Backorder and preorder are frequently misused as well: marking an item preorder when you simply have not restocked it misstates the offer, and preorder items normally need an availability date to be accepted. Variants are a quiet trap, since a style can be in stock overall while the size somebody wants is gone, and a feed built at style level hides that. Finally, a page that returns an error or redirects when a product sells out breaks the check entirely.
How to get it right
Shorten the gap between reality and the feed. Fetch more often for fast-moving lines, and where your platform supports pushing an immediate update when stock changes, use it. Make sure the value is calculated per variant rather than per style.
Keep the sold-out product page live and honest, showing the status clearly rather than redirecting away, so the automatic check has something to read and your organic listing survives. Decide in advance how a restock is handled, since an item flipping between states repeatedly is a sign the threshold is set too tight. Where several channels read the same source, feed the same stock number to all of them, including any catalogue used for dynamic product ads, so out-of-stock handling is consistent everywhere.