Timebase in Pro Tools

In pro tools, material (audio or midi) on a track is associated with a type of Time Scale. All track types can be set to either sample-based (for the Sample Time Scale) or tick-based (for the Bars/Beats Time Scale). Different tracks can be set to different timebases as needed.

Audio tracks are sample-based by default. This means that audio clips have absolute locations on the timeline and are tied to specific sample locations. If you change the tempo or meter the audio will not move. This is helpful, for instance, if you import an audio clip and want to build other audio or midi tracks around it and end up changing tempos or meters a few times. You don't want to affect the original clip. 

However, MIDI and instrument tracks are tick-based by default. This means that midi clips are fixed to bar and beat positions and move relative to the sample timeline when tempo and meter changes. So if you change the tempo, the midi will either speed up or slow down accordingly. 

A good tip to keep in mind is Elastic Audio-enabled tracks can be switched to tick-based in order to automatically follow tempo changes in your session and conform to the session's tempo map. 

And lastly, you select whether a track is sample-based or tick-based when you create it, but you can change timebases later as needed. 

Hope this helps!

Peace - and HEY make it a great day!

Tim