SPECTRUM Performance Manager (SPM) is what schedules and manages the tests against a test host (Cisco Router, SystemEDGE Host with SRM, etc.)
SystemEDGE is an SNMP Agent on steroids.... It is capable of a lot more then a standard SNMP agent (i.e. Net-SNMP). The SystemEDGE agent is extendable with various modules that allow it to monitor/perform various things of which the Service Response Module (SRM) is one of them. The Service Response Module is a testing module that can perform HTTP/HTTPS URL tests, TCP/IP Port Tests, SMTP Tests, etc.... it can really do a lot of things but generally needs something to manage the tests on its behalf.
in the case of SPECTRUM vs SystemEDGE SRM - if SPECTRUM is going to be managing your tests (through SPM) you CANNOT have VAIM managing the Service Response policies. This will cause problems as SPECTRUM attempts to add/modify/delete tests through it's interface.