As the literal meaning of Purge implies is to remove. When MME/SGSN shall remove the data of a subscriber from its (temporary) storage because of following reasons then MME/SGSN shall trigger PUR toward HSS to notify that subscriber is in-active for several days or by MMI (Man Machine Interface) from MME/SGSN.
Before PUR message it was assumed that subscriber is attached atleast once (i.e. Subscriber Profile is downloaded to MME or SGSN) and PUR comes from the same MME/SGSN on which subscriber is attached (i.e ULR is triggered). If it is received form other MME/SGSN then HSS shall return success no need to freeze temporary identity.
When MME send PUR to HSS then HSS marks(set a flag) that subscriber is Purged from MME, similarly for SGSN, HSS marks that subscriber is purged from SGSN. In response HSS shall send an indication to freeze the M-TMSI (Temporary Identity) to MME for some time so that same temporary identity can be used when subscriber became active, similarly to HSS sends freeze P-TMSI indication to SGSN.
When Combined MME-SGSN sends deletes a specific section of data then it shall use PUR-Flag to inform HSS that only MME or only SGSN specific data is deleted.
Following cases shall be performed.
COMBINED MME-SGSN without PUR Flags
COMBINED MME-SGSN Only MME data is deleted.
COMBINED MME-SGSN Only SGSN data is deleted.
Your Comments /Suggestions and Questions are always welcome, shall clarify with best of knowledge. So feel free to put Questions.