The DD statement T0000001 contains the messages issued by worker task number one running in the user server address space. This task gets woken up by the main task to fulfil the requests coming in from clients.
If the parameter NUMTCB in the user server configuration is bigger than one, there is more than one worker task.
During startup, important control block addresses and configuration parameters used are listed. The worker task successfully initialized when this message is issued:
TAU0142I hh:mm:ss.ttt z/Server Subtask READY for clients
After startup it shows which client the worker task was active for. First it acknowledges that it was activated and then it lists what it was activated for. A typical example would look like this:
TAU0023I hh:mm:ss.ttt Subtask for z/Server activated TAU0059I hh:mm:ss.ttt TAUZCAPP ended. RC : 00000000