Harshita, both the procedures defined for S6a interface. S6a interface exists between MME and HSS.
When UE connects to a new MME (due to normal attach, or TAU, or due to handover) . As part of this connection, MME sends Update location Request message to HSS to inform the actual location of a subscriber.
Suppose a subscriber was attached to MME1, due to idle mode mobility it moves to a new tracking area. Now UE needs to inform about this new tracking area to its's core network. It sends TAU request to a new MME. New mme based on the received information from UE, it fetches ue context form old MME and then proceed. After that new MME sends Update location Request to HSS and HSS sends Cancel Location to Old mme to release the context.
This is how Update location Procedure being used.
PurgeUe Procedure is being used by MME towards HSS. Due to a long inactivity of an UE, MME initiates detach implicitly and inform to HSS via PurgeUERequest Command message.
Hopefully above details may help you.
