The ‘Instantiated signal’ field for a Signal-type Event must be defined, whereas the ‘Called operation’ and ‘Expression’ fields must be empty.

An Event is the specification of a specific occurrence at a specific point in space and time.

A Signal-type Event is meant to define a Signal, and cannot trigger an Operation at the same time.

Add a reference to a Signal, if the Event doesn’t have one.

An Event referring to an Operation should be a Call-type Event.