A construct within an interaction that comprises an operator keyword and one or more interaction operands, each of which is a fragment of an interaction. It is shown as a nested region within a sequence diagram.
Interactions include a number of constructs for representing contingent behavior, such as conditionals, loops, and so on. Because many of these constructs share structural information, they are grouped together as kinds of combined fragments. A combined fragment has a keyword that specifies which construct it is.
Depending on the keyword, there are one or more embedded operands, each of which is a structured subfragment of the overall interaction.
A combined fragment may also have zero or more gates, which ...