A connection point in an interaction or interaction fragment for a message that comes from or goes to outside the interaction or fragment.
A gate is a parameter that represents a message that crosses the boundary of an interaction or interaction fragment. Messages within the interaction can be connected to the gate. If the interaction is referenced within another interaction, messages may be connected to its gates. When the interaction is executed, messages connected through gates will be delivered properly.
In a sequence diagram, a gate is simply a point on the boundary of the sequence diagram or interaction fragment that is connected to the head or tail of a message arrow. The name of the connected message arrow is the ...