As part of the Purge UE procedure, HSS receives the Purge UE Request command message from MME/SGSN and responds back to MME/SGSN using the Purge UE Answer message.
Presence of PUA-flag is conditional optional within the Purge UE Answer message.
PUA-flag informs about the "Freeze M-TMSI" or "Freeze P-TMSI" action need to be taken at the MME or SGSN respectively.