Subscription-Id-Type values to consider when looking up subscribers from Gx INITIAL_REQUEST messages. Defaults to IMSI (International mobile subscriber identity) which will work for EAP-SIM and EAP-AKA based authentication. If, for example, PEAP (Protected Extensible Authentication Protocol) is used for authentication, PCEF may use NAI (Network Access Identifier) as the subscription id where NAI is the PEAP user name.