[TF-M] SPM_IDLE

Mention is made to "SPM_IDLE" in the Cooperative Scheduling Rules document:

https://ci.trustedfirmware.org/job/tf-m-build-test-nightly/lastSuccessfulBui...

I'm struggling to understand section 8.3.5 which references SPM_IDLE but doesn't really define it. Is there more info on this topic? It appears to be a proposed solution for allowing other NS threads to be scheduled while the current NS thread is waiting for an asynchronous event in the secure service it has called.

Alan