Can you clarify what you're asking? Are you asking when you should call for additional units to assist?
Also, keep in mind that different agencies use different terminologies, and rescue seems to be one of those. Out here, we don't really use the term rescue by itself, but our FD-run ambulances are all refered to as rescue ambulances (RAs), so if someone says rescue, that's what they mean. What would be called a rescue or heavy rescue on, say, the East Coast, is called a USAR here, and our truck companies handle most of the extrication/light rescue duties.
Out here, the standard response to an EMS call is going to be an engine company and either an RA or a Squad (a non-transport ALS unit...usually a pick-up with a contractor-type body). Freeway TCs usually get two engine companies and an RA/Squad, and if extrication is involved, a truck company will also be dispatched.