-no idea what (single) means
-I think the "variable volume limitation" should be called volume reduction that can be switched off, because you can only switch the thing on or off.
switch the thing on or off. After each start of the unit, the volume is reduced; you can switch off the reduction by pressing the field on the right in the bottom row, where something like an ear is drawn on it.
You first have to hold it down for about 1 second, then release it briefly and press the same button again very briefly, then the reduction is switched off. when reduction is switched on, the volume is reduced quite a lot, so the reduction thing is probably switched off most of the time.
It hasn't jammed so far, but the unit has only been in operation for a fortnight.