Midi defines pitches as numbers between 0 and 127, of which key 60 equals central c, key 69 central a, etc (one key per semitone). This functions well for 12-tone equal temperament, but raises problems for other tuning systems. Not only are many of them not equally tempered, but some of them have other divisions of the octave (e.g. Arab or Indian scales) or don't even have on octave at all (e.g. Bohlen-Piece). In order to accommodate these scales, the Scala application allows to define tuning (.scl) and keyboard mapping (.kbm) files that redefine the Midi pitches 0-127 to all kinds of scales.
Musical Instruments Digital Interface is the standard protocol used by most hardware and software Keyboards and Instruments.