In a UML sequence diagram, what is a formal gate?

In a UML sequence diagram, what is a formal gate?

• A. A guard condition for a fragment
• B. A message that enters or leaves the referenced interaction
• C. A return value from a function
• D. A point where a fragment ends

Answer: (B)

In a UML sequence diagram, a formal gate represents the entry and exit points for the interaction that is being referenced. It defines how messages flow into and out of that referenced interaction, acting as the interface between the caller fragment and the called interaction. So, a formal gate is about where messages enter or leave the referenced interaction, not about a condition, a return value, or where a fragment ends. Guard conditions control which paths run in a fragment, return values are data produced by operations, and the end of a fragment is a separate concept—none of these describe the gate itself.

In a UML sequence diagram, a formal gate represents the entry and exit points for the interaction that is being referenced. It defines how messages flow into and out of that referenced interaction, acting as the interface between the caller fragment and the called interaction. So, a formal gate is about where messages enter or leave the referenced interaction, not about a condition, a return value, or where a fragment ends. Guard conditions control which paths run in a fragment, return values are data produced by operations, and the end of a fragment is a separate concept—none of these describe the gate itself.