As always I stand to be corrected, but as I understand it the M811 is just a chassis/cab.
The M809 is 'standard' wheelbase (179"), M810 is short WB (167"), M811 and M812 are long WB (215").
With that in mind, it could be whatever type of truck depending on what one chooses to mount on it, eg. a generator truck.
Examples would be- an M809 with a cargo bed is an M813; an M810 with a 5th wheel becomes an M818, put a dump bed on it it becomes an M817. M811 with a cargo bed is an M814, with an expando van it becomes an M820. M812 is the basis for the M821 bridge truck.
Cheers