similar to Java, it's an expression returning the current instance ( a FlightStatus in your case ) of the object being matched by the pattern. Two caveats here: "!=" means "not equals" rather than being an identity difference check, and "not exists" is redundant, since the semantics of the not CE is exactly that of "not exists" Davide -- View this message in context: http://drools.46999.n3.nabble.com/Reserved-word-this-tp4020145p4020154.html Sent from the Drools: User forum mailing list archive at Nabble.com.