S-FM = Anyone can host, I thought it was no more than 15 players.
M-FM = Anyone can also host but there can only be 1 M-FM going at a time. To be able to host you have to be elected as host by majority. Up to 20 I guess.

FM = Only moderators+ can host, no limit in player capacity. Only 1 FM can be going at a time. FM has more time in days and night, sometimes it's mechanics are more complex. FM also has more experienced players and doesn't happen as often as M-FM or S-FM.