This duplex mode allows a repeater (rather than app_rpt) to perform the repeat function. This is also known as in-cabinet repeat. Duplex Mode 3 is unique to repeaters and is not likely to be used on simplex, half-duplex nodes or hotspots. Setting duplex=3 in rpt.conf prevents Rx repeater audio from being sent to the repeater Tx. However, courtesy tones, morse code, voice messages and link audio are still sent to the repeater.
Duplex mode 3 might be used with a repeater where in-cabinet repeat cannot be disabled.
TO BE CONTINUED... Notes to self: No audio delay. RTCM duplex mode 3.