Hi
U-zone and TR-zone are just bins in 922 storage type, these bins are pre-configured by SAP in WM movement types as source/destination. You may create your own bins in 922 storage type and replace standard bin.
What I can see is U-zone is used in WM movement type 453 (Posting change, returns) and TR-zone is used for other posting changes like stock to stock movement, sloc to sloc or plant to plant movement. SAP might want to provide possibility to bifurcate stock in 922 storage type for users identification
BR
Kamlakar