Tracking area update procedure is used by UE to updated its current location (LTE tracking area level) to its registered MME/Core Network. There are two ways to do it. One is known as "periodic TAU" and the other one is "event based(a periodic) TAU".
During attach and tracking area update procedure, MME provides a list of tracking areas to UE.
If UE detects a new tracking area against the list provided by network, it triggers tracking area update procedure. It is known as event based tracking area update.
