PM-OPR-0020

Every Operation MUST be either a Transaction, triggered within the domain, or an Action, involving an Event crossing the domain boundary.

Non-normative: this is the distinction that makes PM-EVT-0010 survivable. If every Operation needed an Event, a domain could not take an internal step. The description also mentions a third kind in passing — a "Main" Operation that achieves no State and exists only to parent a taxonomy — which is why PM-STA-0040 carves it out.