Halliday & Matthiessen (2014: 410-1):
As we have seen, the verbal group is highly grammaticalised: all elements of its structure except for the Event are realised by grammatical items, the Event being the only one that is realised by a lexical item. It follows that the system network of the verbal group is a network of systems representing contrasts that are purely grammatical in nature. The only system that extends in delicacy towards distinctions that are realised lexically is the system of event type — the verbal group analogue of the thing type system in the nominal group. This system is concerned with distinctions among verbs relating to their temporal properties (thus complementing the clausal system of process type, which is concerned with distinctions among processes relating to configurations of process plus participants).