Old Noob question...MM/MC

Help me out. I don't remember what this is for...My Denon PMA-900V has a 4 way switch...MM-MC/subsonic-MM-MC. I think it's for differant cartridges....right?

MM = moving magnet cartridge. Output generally several millivolts
MC = moving coil cartridge. output generally a fraction of a millivolt

The subsonic filter is to filter out infrasonic noise from the turntable/record/stylus/suspension resonant system; keeping it from wasting your amps power and abusing your speakers.