The Koha manual spells out the available ILL statuses, but the information is very bare bones. It is not clear which ones are mandatory, which can be skipped if not applicable to a library's workflow, what triggers some of them beyond the obvious like 'checked out,' or even the expected sequence of statuses in the lifetime of a request (is completed before checked out? that's confusing in the manual without explanation).
Thank you for flagging this. If you have content for the case you describe, the Documentation team would be happy to help you format it for submission to the Manual :-)