Internal Sync/MIDI Clock Sync/ReWire Sync

These three options are used to specify which type of tempo synchronization you prefer:

Internal Sync

When this is activated, the program is not synchronized to any external source. It plays in the tempo set on the transport panel.

MIDI Clock Sync

When this is activated, the program is synchronized to external MIDI Clock, as set up in the Preferences dialog. The tempo setting on the Transport is of no relevance, Reason plays in the tempo of the incoming MIDI Clock signals.

ReWire Sync

When this is activated, Reason is synchronized to another application via ReWire. This is not a setting that you can activate yourself, it is automatically enabled when the program is in ReWire slave mode.