The SSI is the interface used by routines (IBM®, vendor, or installation-written) to request services of, or to pass information to, subsystems. An installation can design its own subsystem and use the SSI to monitor subsystem requests. An installation can also use the SSI to request services from IBM-supplied subsystems. The SSI acts only as a mechanism for transferring control and data between a requestor and the subsystem; it does not perform any subsystem functions itself.