Sentence:  For the occasion on which everyone already knows everyone else and the host wishes them to meet one or a few honored newcomers , then the `` open house '' system <WSD>is</WSD> advantageous because the honored guests are fixed connective points and the drifting guests make and break connections at the door . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)