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

Uncheck it to also let the user type entries that are not in the list, each submitted verbatim as its own element. This is a partial workaround for paginated REST APIs whose desired value falls outside the first page returned to the plugin (see #61 and #86). The endpoint is then not contacted to validate a submission.

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