BBT markers allow resetting the timeline to a particular bar and beat. Most of the time a BBT marker will reset it to 1|1|0 (the very first beat of the very first bar) so that for each song in a multi-song session there would be a musical time origin. However it's entirely possible to reset to an arbitrary value such as 7|3|0.

To create a new BBT marker, -click on the Bars:Beats ruler. This will open a dialog titled 'New Music Time' where you can name the marker and define what bar (left field) and beat (right field) this marker should reset the time to.

New Music Time dialog
New Music Time dialog

The timeline grid and the ruler will be updated accordingly:

The BBT marker resets the musical time
The BBT marker resets the musical time