The B means Base version and T means Target version in the column called ZDU version. It designates of which process, old or new, the Task depend. The goal is that all processes switch from B to T during the ZDU.
The simple rules are:
Even after the the ZDU is finished there are some Tasks in the Activities / Process Monitoring that still have a T in the ZDU version column.
Component : Automation Engine
Development has confirmed that once the ZDU has finished, the ZDU Version (B/T) column has no impact anymore whatsoever. So this is definitely a safe situation.