Basically if a wheelchair and/or
ECV can navigate a queue all the way to boarding the line is considered "mainstreamed" and the person in the chair will be in line just as anyone else.
If due to width, turns, stairs or any other reason a chair cannot get to the boarding area then an alternate entrance is used. Usually the alternate entrance tends to be the exit.
Quite often use of an alternate entrance can take longer than going through the regular line.
Under the ADA the facility (Disney) is not permitted to ask for proof of a disability. Some of us might like to see some proof requirement for people to rent a wheelchair or ECV, but that does not happen.
The normal Disney response for stamina or mobility problems is recommending rental of a wheelchair or ECV.
A GAC usually is given based on other (invisible) disabilities. No note is required, but if one is used it should indicate needs and not diagnoses.