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, Windows provides MIDI authoring guidelines. These guidelines include a list of standard patch assignments and standard key assignments 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.