In sequence diagrams, a combined fragment is a frame that defines the interaction fragments and that describes the traces in an interaction. Interaction fragments refer to the event occurrences, execution occurrences, interaction occurrences, and combined fragments in sequence diagrams.
You create a combined fragment to represent a construct in a sequence diagram. You can represent different types of constructs, such as optional, alternative, parallel, loop, and so on. You can group the messages and interaction fragments that are involved in the interaction in a combined fragment to compact and simplify complex interactions.
As the following figure illustrates, a combined fragment is displayed as a frame that covers lifelines and contains interaction operands.