This is handy because analog keyboards have a tendency to drift out of tune over time. Song Select allows the user to specify to other units which song will be played next. Song Position Pointer is a way to keep two machines synced to the closest sixteenth note. It does this by sending out a value that represents the number of sixteenth notes that have passed since the beginning of the song. MIDI Time Code is another form of external sync that, when on, is constantly sent out of the device and represents absolute time.
Absolute time tells the exact position of the song at every moment of transmission.
Within this group, there is MIDI Clock, Start, Stop, Continue, Active Sensing, and System Reset. MIDI clock, when engaged, sends out 24 pulses for every quarter note that passes. Combine this with Song Position Pointer messages and you have MIDI Sync, which is an external sync mode that slaves can respond and lock to. Next, the Start, Stop, and Continue messages obviously control the playback of the song. Active Sensing is very important for live performances because it sends a pulse out every 250ms that the slaves receive and become addicted to.