M - Internal relays - auxilliary relays used by the cpu and are not latched (cleared at power failure or cpu reset)
L - Latch relays - auxilliary relays used by the cpu and are latched (not cleared at power failure)
F - Annunciators - Internal relays convenient for equipment fault / failure detection
V - Edge relays (q cpu) - similar to a one-shot
B - Link relays - CPU devices used to refresh LB relays in a melsecnet network
S - Step relays - relays for step instructions in sequential function chart
(Copied from mrplc.com)