Section: 19.7.2 External Identifier
Content: An External Identifier identifies a subscription associated to an IMSI. A subscription associated to an IMSI may have one or several External Identifier(s). The External Identifier shall have the form username@realm as specified in clause 2.1 of IETF RFC 4282 [53]. The username part format of the External Identifier shall contain a Local Identifier as specified in 3GPP TS 23.682[ Architecture enhancements to facilitate communications with packet data networks and applications ]  [98]. The realm part format of the External Identifier shall contain a Domain Identifier as specified in 3GPP TS 23.682[ Architecture enhancements to facilitate communications with packet data networks and applications ]  [98]. As specified in clause 4 of IETF RFC 4282 [53], the Domain Identifier shall be a duly registered Internet domain name. The combination of Local Identifier and Domain Identifier makes the External Identifier globally unique. The result of the External Identifier form is: "<Local Identifier>@<Domain Identifier>" An example of an External Identifier is: Local Identifier in use: "123456789"; Domain Identifier = "domain.com"; Which gives the External Identifier as: 123456789@domain.com