What is being signaled when you hear 5 short blasts from another vessel’s horn?

Five short blast from another vessel's horn signify DANGER OR DOUBT. When two vessel are about to pass each other by, they have to communicate each other intentions in order to avoid collision. If one of the party does not understand the message that is been relayed or feel that the other party is not taking enough caution to avoid collision, then he should sound the five short blasts to signify that he does not understand what the other party is trying to do.