Off: The region doesn’t follow changes to the project tempo, and isn’t Flex enabled.
On: The region is Flex enabled, so it follows any manual Flex edits. It also follows any changes to the project tempo.
Bars (On + Align Bars): The region is Flex enabled, and Smart Tempo beat markers are used to conform the region to the project tempo at every downbeat (Smart Tempo overview. For more information about the Smart Tempo Editor, see PDF
Control Surfaces Support Guide: PDF