When checked (the default), the build form offers only the entries returned by the REST endpoint, and every selected entry must match one of them when the build is submitted. The endpoint is contacted again at submission.

Uncheck it to let the user add entries that are not in the list: typing into the multi-select offers the typed text as Use "<text>", and picking it adds the text verbatim as its own element. Any non-empty element is then accepted, and the endpoint is not contacted at submission.

This helps when builds must keep running while the endpoint is unavailable, and when a value the endpoint does not (yet) return should be passed by a trigger, the remote API or the build form.

Whether an empty list is accepted remains controlled exclusively by the separate Allow Empty Value option.