'event' occurs when something happens in space,
for example when part(s) of state change a value to a value belonging to certain set.
in response to 'event occurance' parts of code might be executed, do something, possibly causing certain conditions to cease it's hold & new conditions to arise, to hold.
before this is possible, parts of code must 'register' for being informed about event's occurance.
event notification(s) might be a cause for something to happen, or for condition(s) to arise, or for something to cease happening.
that's why conditionality is often joined with event-based programming.
Conditionality & Events are part of Stitie Space 1.1 'Sunsail', are methods for Agile Transformation of Information State.
See also, if You wish: Conditional Software Tiering, Causes & Conditions.