MIDI beat clock
MIDI beat clock is a way to ensure that several synthesizers stay in synchronization. It is not MIDI timecode.
Unlike MIDI timecode, MIDI beat clock is sent at a rate that represents the current tempo. It is used to maintain a synchronized tempo and measure position for synthesizers that have BPM-dependent voices and also for arpeggiators.
Because of limitations in MIDI and synthesizers, devices driven by MIDI beatclock are often subject to drift. For this reason, it is a common practice on equipment that supports another clock source such as ADAT or wordclock to use both that source and MIDI beatclock.