QHT is used to retain unused quota (volume and time) on the P-GW for a short period (typically 5 - 60 seconds). The timer prevents excessive quota negotiation signalling when traffic of a particular category within a given PDP context repeatedly resumes after only a short break.
The QHT is started when the first granted quota is received by the P-GW. The timer is restarted every time a service request is forwarded. When the QHT expires, the P-GW considers the instance as no longer in use, closes the category instance and notifies the OCS. The CCR message reports the used quota and sets the Reporting-Reason AVP to QHT.