Transport commands can be customized with Preferences > User Interface > Edit Keyboard Shortcuts and configured with Playback > Audio/MIDI Setup > MIDI > Remote Transport Control.

Transport Bar

Rewind: Reset playhead to the very beginning.
Stop: Stop all playback.
Pause: Halt playback at current position. Click again to continue playing from there.
Play: Start or restart playback (Space). If something is selected on the Time Ruler or in a Parameter View, playback starts from there.
Solo: Start playback, rendering only the currently selected tracks. This is different from soloing audio output with the S button.
Preview: Listen to the currently selected object by rendering an example phrase.
Record: Record the currently selected phrase from external MIDI input. See: Recording.
Record Parameter: Record anything from external MID input and convert it to the currently selected Parameter. See: Recording Parameters.
Loop: Run next playback in cycles as marked on the Time Ruler.
External Sync: Transmit TEMPO, LOCATE, START, STOP commands to external hardware in synchronization with local playback. See: External Synchronization.
Overdub: Don't clear the phrase before recording, i.e. add whatever is recorded to the existing phrase.
Bounce to Disk: Save audio output of next playback to a file. The file is placed next to the arrangement document (File > Show File in Finder). Requires all instruments to use the same Audio Engine.