Transport

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. Hold down Alt to play only instruments with phrases in the current container.
Solo: Start playback, rendering only the currently selected tracks. This is different from soloing the audio signal with the S button.
Preview: Listen to the currently selected object by rendering an example phrase. You can also use this to preview Harmony or any other parameter that's currently selected. Enable Playback > Tie Auto-Chord Notes to sustain all notes that are already playing in the previous chord.
Record: Record the currently selected phrase from external MIDI input. See: Recording.
Record Parameter: Record anything from external MIDI 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.
Select the root container to create an audio file for the entire arrangement. You may optionally select a starting position on the timeline. Recording will stop at the end of the root container, so it is a good idea to leave have a little room at the end for all sounds to settle.
Select one or more child containers to trim the captured audio file exactly to their bounds. Stop playback after all containers have been played.
Press Play to begin the recording and Stop to end it.