The MTP 3 data interface supports one or more applications using service access points (SAPs). One SAP is defined for each application that uses the MTP 3 service. An application binds to a particular SAP at initialization time, specifying the SAP ID to which it wants to bind. Each SAP is associated with a service information octet (SIO) value, which in turn identifies the upper layer protocol in use on that SAP (for example ISUP, TUP, SCCP). Therefore, only one application process can handle incoming messages for a particular upper layer protocol (only one process receiving incoming ISUP messages).
The MTP 3 configuration file specifies the number of SAPs (MTP 3 users or MAX_USERS) and the characteristics of each SAP. For more information, refer to Mtp3InitNSapCfg and the NMS SS7 Configuration Manual.