Sitemap
A sitemap is a machine readable list of the URLs on a site, published so that a crawler or an agent can enumerate what exists rather than discovering it by following links.
For a store the useful distinction is what it lists. A sitemap that stops at collection pages says that you sell things without saying what, so the catalogue still has to be crawled page by page to be understood.
It is the minimum rather than the whole answer. A sitemap tells an agent your product pages exist; a feed tells it what they contain, which is the difference between being enumerable and being cheap to evaluate.
Why it matters to a store
Being expensive to enumerate is a soft form of invisibility. You are technically present and practically skipped, and nothing reports that as a failure.