SysEx (System Exclusive)
MIDI's escape hatch: a message whose contents are entirely up to the manufacturer.
A SysEx message starts with the byte F0, followed by a manufacturer ID, any number of data bytes, and ends with F7. Everything in between is Roland's or Yamaha's business, or whoever made the box. That let MIDI stay small in 1983 while still giving every synth a way to dump its sound banks, receive parameters the standard never anticipated, or update its firmware.
Standardised commands ride along too: "GM System On", which switches a module into General MIDI mode, is a universal SysEx message. The firmware angle is exactly why the Web MIDI API asks for separate permission before allowing SysEx — a web page free to send arbitrary SysEx could, in the worst case, rewrite a synth's software. MIDI 2.0 keeps the concept, with 8-bit data.