Abbie, sure, so here's my understanding so far:
An "OPES system" describes the collection of all OPES entities (i.e. OPES processors and callout servers) that are operated by a single provider (or, alternatively: ...within a single trust domain).
If this describes what an "OPES system" was meant to be, I somehow don't like the term too much. Maybe the term "OPES domain" would be appropriate?