If you use one, it's a frequency much lower than the carrier and well above the baseband, which is modulated with the baseband and then mixed with the carrier. The result is that you get a sideband at [carrier+(IF+modulation)] and one at carrier-[(IF+modulation)]. That allows the transmitter/receiver to do the rough work at the carrier frequency, focusing on isolating a clean, largely undistorted signal, then do the tricky business of isolating the baseband signal from the IF at a more comfortable frequency without having to process things at multi-GHz rates. Not all transmitter/receiver schemes use an IF but it is pretty popular.

RE