From doing some quick reading (instead of work, lol) it sounds like as long as there is a human filling the job as President, when the VP steps in it must be in that Acting capacity. So the President could notify Congress ahead of time that he will be temporarily incapacitated due to a medical procedure, and the VP will act as President for that period of time.
Or, if the President becomes incapacitated without giving that advanced notice, the VP (along with a majority of the President’s cabinet) can notify Congress of that fact, and the the VP can assume the Acting President role. Then the President simply informs Congress when he is better again. If there is any dispute over that between the President and VP, Congress gets to ultimately decide whether the President is still incapable, although it would take a 2/3 vote of both Houses to render him incapable.
There seems to be absolutely no difference in terms of the powers of “Acting President” vs. “Actual President”. However, since the Acting President is normally a temporary role, one would think that they wouldn’t be signing legislation or making appointments unless the President is incapacitated for a long time, since that would piss off their boss.
Dick Cheney just wrote letters to his grandkids while he was Acting President, just to be able to sign them “Acting President”.