The innovative part of MOD files was that you
were not limited to using your computers typical FM-
style synth; instead, you have near-infinite composing
freedom due to the fact that you use your own custom
samples.
Original MOD files had 15 or 31 sample slots and
4 channels.
IIRC, you can spot a MOD file by the "M.K." signature at
around offset 1080.