Section: 7.3.21 UE Registration Query Request
Content: The direction of this message shall be from S4-SGSN to MME (see Table 6.1-1). This message shall be used to support CS/PS coordination for shared UTRAN and GERAN access. When an S4-SGSN receives a UE Registration Query from a RAN node, including an indication to also query MMEs, and if the UE (identified by IMSI) is not registered in the S4-SGSN, the S4-SGSN shall send a UE Registration Query Request message to all MMEs that may hold the UE's context, as specified in the clause 7.1.6 of 3GPP TS 23.251[ Network sharing; Architecture and functional description ]  [55]. NOTE: How the S4-SGSN determines which MMEs it will query, is based on local configuration. Table 7.3.21-1 specifies the presence of IEs in this message. Table 7.3.21-1: Information Elements in UE Registration Query Request