What notation is used to signify a combined fragment in a sequence diagram?

The notation used to signify a combined fragment in a sequence diagram is represented by a rectangle placed over one or more lifelines. This rectangle visually indicates the boundaries of the combined fragment, allowing for the organization of related interactions within a sequence. The use of a combined fragment is essential as it encapsulates logic such as alternatives, loops, or interactions that are conditional or iterative, which are fundamental concepts in modeling the behavior of systems.

When you see a rectangle that extends over multiple lifelines, it implies that the interactions occurring within those boundaries are not just linear but may involve complex conditions or repetitions. This notation effectively helps in understanding how different parts of the system interact under specific circumstances, thus providing clarity and structure to sequence diagrams. Understanding this notation is vital for accurately representing system behavior in UML diagrams.