Page "MIDI" Paragraph 63

Audio effects units that are frequently used in stage and recording, such as reverbs, delays and choruses, can be remotely adjusted via MIDI signals.

Some units allow only a limited number of parameters to be controlled this way, but most will respond to program change messages.

The Eventide H3000 Ultra-harmonizer is an example of a unit that allows such extensive MIDI control that it is playable as a synthesizer.