In sequence diagrams, combined fragments define conditional structures
in interactions.
A combined fragment defines an expression of interaction fragments. A combined fragment is defined by an interaction operator and its corresponding interaction operands. You can use combined fragments to describe several traces in a compact and concise manner.
A combined fragment consists of an interaction operator, one or more interaction operands, and an optional guard condition.
As the following figure illustrates, a combined fragment is displayed as a frame that covers lifelines and contains interaction operands. A combined fragment separates the contained interaction operands with a dashed horizontal line between each operand.
