DLS is short for Downloadable Sounds and is an instrument format created by the MIDI Manufacturers Association. The format is supposedly public, but the version 2.0 and higher specification does not exist in electronic form and must be purchased. The older version 1.0 specification is available for download though. The synthesis model is very similar to SoundFont files but has more flexibility in some areas.

Resources

DLS specification - Documents on the DLS format (no downloads for version 2.0+)