An account id names one connected LinkedIn account. You get one back when a sign-in completes, and you put it in almost every call afterwards — in a path segment, a query parameter, a JSON field or a multipart field, depending on the operation.
It is opaque. It has no prefix, it is not a UUID, and it carries no meaning you should parse. Treat it as a string you store and send back.
Why it answers 404 and not 403
Every subscriber on the Premium plan is forwarded to one shared upstream tenant, which means account ids from every customer live in the same namespace. Nothing reaches that upstream naming an account id until we have confirmed that your organisation connected it.
A miss answers 404, never 403. The difference is not politeness. A 403 would mean the id exists and is not yours — so somebody could sit there generating ids and tell real ones from fake ones by which answer came back. A 404 for both says nothing at all.
This is checked in all four places an id can arrive: the path, the query string, a JSON body field and a multipart field. An id that reaches the upstream unchecked would be a cross-tenant read.