This document defines a new IMAP untagged response code, "INPROGRESS", that provides structured numeric progress status indication for long-running commands. I found the document to be clear and easy to understand. The needed IANA instructions are included and clear.

My only suggestion would be to describe expected behavior in cases where a BCP 14 directive is not met. For example, the text says that "GOAL: a number indicating the total number of items to be processed.  The number MUST be positive and it SHOULD NOT change between successive notifications for the same command (i.e. for the same cmd-tag)." What should be done if a GOAL value is negative, or of it does change between successive notifications for the same command? It might take no more than a single sentence somewhere to describe the action to be taken when one of these BCP 14 directives is violated.