The MIDI protocol consists of MIDI events that carry four types of messages: 1) voice messages; 2) system common messages; 3) system realtime messages; and 4) meta messages.
While MIDI voice messages are specific to a MIDI channel, system common and system realtime messages are not specific to a channel and thus all MIDI devices on the MIDI system should respond to such messages. Thus, these messages are "system" messages. Some of these system messages provide timing information and usually require that devices react in real time, and are called "system realtime" messages. The rest are "system common messages".
MIDI system common messages are those MIDI messages that prompt all devices on the MIDI system to respond (are not specific to a MIDI channel), but do not require an immediate response from the receiving MIDI devices.
The following messages are MIDI system common messages.
Musical Instrument Digital Interface (MIDI)