The other two terminals connect to the inductive loop transmitter. Inductive loop is a very simple loop of wire usually (just one turn) that is able to inductively transmit the audio to a tele-coil equipped hearing aid. The tele-coil looks very similar to a tiny version of an AM antenna. A tiny solenoid wound on a ferromagnetic core. Inductive loop audio transmission is a widely used wireless transmission technology in hearing aids. Apple makes it a little mysterious by calling it an "HAC radiator".