If any past event evaluates to true with the current event for the simple boolean expression, the output set is the incoming event plus all matching past events.
If no events in the window match the current event for the simple boolean expression, the output set is empty.
If a window is the last or only operation of a correlation rule, then the output set of the window is used to construct a correlated event (the correlated events being the window operation output set of events with the current event first).