The “MIDI 1.0 Detailed Specification” does not define any standard patch assignments for synthesizers. Therefore, when you create a MIDI file, it won't be reproduced correctly unless it is played back on the same MIDI synthesizer setup used to create it. For example, if you create a piano concerto for your Yamaha DX7 and try to play it back on a Roland LAPC-1, it might be played with a flute instead of a piano.
To enable MIDI files to be a viable format for representing music in multimedia computing, the Multimedia extensions provide MIDI authoring guidelines. These guidelines include a list of standard patch assignments and standard key assign-ments for percussion instruments. Using the MIDI Mapper, MIDI files authored to these guidelines can be played on any multimedia computer with internal or external MIDI synthesizers.