FM Radio Mode (FM) Receive Bluetooth from phone and transmit FM to car or home stereo Bluetooth Receiver Mode (RX) - Transmit Bluetooth Audio from a phone to the BT Freq which sends analog audio to car, home or wired headphones using 3.5mm auxiliary input Bluetooth Receiver Mode (RX) Receive Bluetooth from phone and send analog audio to 3.5mm headphones or other analog AUX input Bluetooth Transmitter Mode (TX) - Receive audio from a device via. 3.5mm aux cable to BT Freq that then transmits via Bluetooth audio to a Bluetooth enabled device Transmit Bluetooth Audio from a phone to the BT Freq that then will transmit the signal via FM radio to a car's stereo