This connection is used for both Mic Input and PTT switch. If R68 is installed this provides phantom power to the mic for using an electret mic, without this resistor installed a typical dynamic mic can be used. This has to be selected in "Mic/Line Select" option or by M3 (long press). The level can be adjusted by "Mic Input Gain" option or using MIC.
All mcHF have a clunck that is an electronic noise, when pushing PTT which is consequence of the applying bias to the electret mics. This can be solved applying microphone bias all the time. See Recommended Modifications for more information.