>>312121951
Inside out tracking uses cameras on the headset to track the headset and controllers. This is used on WMR (G2) and the newer Oculus headsets. Outside in is the opposite and uses external sensors, and is used on the Vive, Index, and Oculus CV1.

Outside in generally has better tracking, and can track behind you. Also if you're using a Vive or Index headset and their base stations you can easily get tracking pucks if you care about FBT for VRChat.