What's the difference, operationally, between an "acting" official and one that has been..

[PoliticAverse]

10. It is longer than 180 days...

Term of office
Once a vacancy occurs, the position is eligible to be filled by an acting officer for 210 days from the date of the vacancy, as well as any time when a nomination is pending before the Senate. If a first or second nomination is rejected by the Senate or withdrawn, it activates additional 210-day periods from the date of the rejection, but this does not apply to a third or later nomination.

If an office remains vacant after 210 days after the rejection, withdrawal, or return of a second presidential appointment nomination, it remains vacant until a person is appointed by the President, by and with the advice and consent of the Senate. In such instance, only the head of an executive agency may perform office functions until such appointment is made in the case of an office other than the office of head of an executive agency.

This period is modified around the time of a presidential transition (when a new president takes office), effectively extending the 210-day limit to 300 days.

https://en.wikipedia.org/wiki/Federal_Vacancies_Reform_Act_of_1998#cite_note-:0-2