I have a Sennheiser wireless system.
On the receiver there is an adjustable AF adjustment.

Set  @ 0db is line level  set  @ -24db is mic level

Wireless what? Mic? Guitar?

Line level doesn't have any additional gain.

Mic level assumes a mic pre-amp has been applied.

Sound systems have several gain stages.  The system could be a guitar/pedal board/head/cabinet, a mic/mixer/amp/loudspeaker, or whatever.  The point is is that most systems are comprised of several different components, and the gain controls on each component must be set correctly in order to achieve a workable gain structure.
The adjustable AF out allows you to compensate for disparities in the output of the audio source, and to ensure a proper level at the input stage of the guitar amp, mixer, or whatever it is that you're running your wireless system into.
If I attach a lavalier to a person who is speaking softly, I might set the AF out at 0 dB, so that I have healthy signal going to my mixing board.  If the lavalier is used on a person who is shouting or speaking loudly, I might set the AF out to -10 or -20 dB.  I set the output lower in the latter scenario in order to avoid clipping the mic preamps on the mixer, in other words, sending the mixer a signal that is too "hot."
We have gain controls such as adjustable mic preamps, pads, and selectable AF outs in order to compensate for the various levels presented to us by performers and their gear.  Watch your meters, understand the equipment, and use your ears to determine the correct gain structure for your system.  If each guitar pickup presented the same output level, or each person sang at the same volume, we could skip the potentiometers, buttons, and switches.  That isn't how it works, though, hence the various gain controls found on every piece of sound equipment.