In UML, combined fragments are used to model complex sequence flows by grouping a set of interactions together. Each type of combined fragment has a different semantic use.
The 'assert' combined fragment specifies that the sequence of events inside the fragment must occur exactly as shown, with no deviations or alternatives. This implies that all possible valid traces in the sequence are exactly those specified within the 'assert' fragment; there are no optional or alternative flows.
References include:
* UML 2.5 Specification (formal/2017-12-05), specifically the section on Combined Fragments and their semantics.
* UML Sequence Diagrams: Overview of Graphical Notation, which discusses the purpose of different combined fragments.