First let me tell you a bit about the TAU procedure.
As part of Attach, MME sends a TAI list (list of Tracking areas) to the UE - For eg.- TA1, TA2, TA3 and TA4.
In lay man's term - MME says to the UE that you can move around in TA1 to TA4 and I can provide you
the required services. Now if the UE moves to a new Tracking area, for eg.- TA5 which is not in that TAI
list, it is the UE's responsibility to inform the same to the MME to continue receiving the service.
The UE does so by the Tracking Area Update Procedure. Now coming to your question which has already been answered by Veer.
The target enodeB may have a TA5 or any TAC which is not in the list of TAI that MME shared with UE during the attach. So the moment UE realizes that it has entered a new TA which is not present in the TAI list, it notifies the MME by the TAU procedure.
If the MME is capable of serving this new TAC ,i.e, TA5, it shall accept the UE's request.
You can refer the link below for complete call flow along with the explanation -