The first concept that needs to be understood is the difference between absolute and relative time . Absolute time is always the same and you cannot speed it up or slow it down.
Relative time is a reference to a musical piece that has an inner tempo.
An advanced MIDI sequencer is able to work with both absolute time and relative time and make adjustments when there are changes in the relative time of a composition. In the chart below the upper line represents absolute time and includes an example of 17 seconds of time. Notice that 2 measures have passed at 4 seconds with a tempo of 120 bps, but it would take 8 seconds for the same two measures at a tempo of 60 bpm and 16 seconds for the same two measures at a tempo of 30 bpm. Advanced sequencers will also subdivided each MIDI clock twenty times for a resolution of 480 times per quarter note. An F8 status byte is a system real time message that is used in a MIDI sequence to connect all the different rhythm values.