In Pro Tools | First, editing and arranging is done on tracks against a Timeline.

Clips and events can be anchored to points in absolute time, or relative time, as measured on the project Timeline. The Timeline includes two different types of rulers for measuring time: Timebase rulers and Conductor rulers.

The Timebase rulers can measure time in two different ways: sample-based time (absolute time) and tick-based time (relative time). For example, the Minutes:Seconds ruler measures absolute time and the Bars|Beats ruler measures relative time.

The relative time of the Bars|Beats ruler is determined in part by tempo and meter. Tempo and Meter rulers reside in Conductor tracks. Tempo and meter events affect the timing of tick-based tracks, and also provide the tempo and meter map for the Bar:Beat grid and Click. You can edit tempo events in the Tempo ruler or Tempo Editor.