I am trying to adapt a CB "roger beep" mic for ham radio and there is
some odd terminology for CB mic wiring.

The CB'ers call "PTT" "transmit (TX)"

They call "audio-out" "audio" (not too bad)

They have a separate line for "receive (RX)" which some call "receive
control." What is that for? Anyone know? By testing, I know it is
not for a speaker in the mic. It shows 6 ohms to ground.

Ken KC2JDY