The specification of both an ontological category as well as a property (technically captured in the UER in form of a type-expression and an attribute) rules out that SLEEP, as an eventity, or a stone, as an inanimate ineventity, could be potential undergoers of the WAKE UP 1 eventity.
the participant class specification works like a filter on the set of all entities, ruling out those entities which cannot be participants of the eventity in question because they do not fulfil the required characteristics.