With SM2, a user can change the due date of a card, and optionally adjust its interval at the same time. e.g. change it from being due in 10 days to being due in 3, and optionally make subsequent intervals be based on an interval of 3 instead of 10.

With FSRS, the interval is ignored and stability is used instead, which means "3!" and "3" currently do the same thing as far as FSRS is concerned.

Originally reported on https://forums.ankiweb.net/t/set-due-date-hide-set-interval-to-same-value-if-fsrs-is-enabled/53890