Arranging : Time, Tempo, Meter, Key, and Chords : Tick- and Sample-Based Timebases

Tick- and Sample-Based Timebases
For more information, see the following topics:
Pro Tools lets you set any track timebase to either sample-based or tick-based.
Audio tracks in Pro Tools are sample-based by default. This means that audio clips and events are located at specific sample locations, and they do not move from that location if the tempo changes in the session—even though their bar and beat locations do change.
MIDI and Instrument tracks in Pro Tools are tick-based by default. This means that MIDI clips and events are located at a specific bar and beat locations, and they do not move from that location if the tempo changes in the session—even though its sample location does change.