Up

– This parameter lets you choose the upward (local) axis. Blender will try to mantain it aligned with the Global Z axis as much as possible.
– As on many other occasions some combinations make no sense (e.g. the same axis choosen as tracking axis and ponting upward), so don’t panic!

– Look at Suzanne:

Suzanne with Axis

– In the left video Suzanne’s Z local axis was set as Up axis so her head points upward.
– In the right video Suzanne’s Y local axis was set as Up axis so her left ear points upward.

Blender Track To Axis Up