The RTL-FM receiver uses the RTL-SDR and enables voice reception on AM, NBFM and Wideband FM across the RTL-SDR's frequency range.
The RTL-FM Receiver is selected from the "RTL-FM Receiver" button on Menu 2. This brings up the receiver menu.
If a USB audio dongle is connected, the audio output is from the USB Dongle (this allows continued DATV transmissions without a reboot). If no USB Audio dongle is detected, the audio output is from the RPi 3.5mm jack. In this case, the RPi needs rebooting before it can transmit.
Allowing for the RTL-SDR frequency offset
If your RTL-SDR has a frequency offset, you can compensate for this by setting a "ppm" value from the "Set ADF Ref Freq" menu (selected from Menu 3).