TAU means "Tracking Area Update" which is used in LTE which is known as RAU i.e. Routing Area Update in GERAN/UTRAN/3G. These procedures are initiated by UE when it detects a new tracking area or routing area.
Whenever UE moves from 3g to 4G-LTE then TAU procedure is initiated, but could not understand your query fully so dont know what to answer. Please check 23.401 or provide input so that it can be answered.