The MUS format is a slightly altered MIDI format, created for DMX and used in the Doom engine soundtracks. MUS files are nearly identical to MIDI files, only slightly more compact. The file format has a size limit of 65536 bytes.
Support for standard MIDI files in vanilla Doom was added in version 1.5, which meant WAD authors no longer needed to convert MIDI files to MUS. However, since this was achieved by the game running MIDI2MUS internally when needed, authors looking for vanilla compatibility should make sure that the MIDI file they include can be successfully converted.
The MUS format can only use the first 9 MIDI channels using the first 8 for music instruments and the 9th for drums. The reason this is forced upon the format is that older soundcards had an FM sound chip with only 9 FM channels. Although some at the time had 18 channels, these were less common.
The MIDI2MUS converter does not create perfect conversions; sometimes the created files can sound distorted.
The MIDI format uses the value 127 as the maximum possible volume in volume change events; MUS allows for higher values. A MUS->MIDI converter that does not take this into account might end up glitching the volume on certain channels. "Smells like Burning Corpses" is known to be affected by this problem.