The MUS format is a slightly altered MIDI format 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.
The MUS format can only use the first 9 MIDI channels; using the first 8 for melodic 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.