A 'WHCS' is a typo for 'WCHS' meaning
"passenger is able to walk but unable to ascend or descend stairs".
It is one of several categories of people whose mobility is challenged and who request assistance in getting all or part of the way to the plane.
(You could have researched this yourself, to be quite honest, isn't it?).

At FRA, at the type of gates you mention, the helpful assistant who comes to collect a WCHS passenger from the plane, usually with a wheelchair, will accompany the passenger in this elevator, using a key issued to that assistant, and then continue to take the passenger to the connecting gate or exit or transit lounge.
In my experience, when asked politely and respectfully, that assistant will usually allow another passenger who is unable to take the stairs but has not requested assistance prior to travel, to come along. However, the assistant is within her/his rights to decline such a request.

Paul