What do combined fragments introduce to a sequence diagram?

Combined fragments are a key feature in sequence diagrams that facilitate the inclusion of control logic, which allows for more complex interactions to be represented clearly and effectively. When you use combined fragments, you can introduce various control structures that enable the modeling of conditional behavior (like "alt" for alternatives), iteration (with "loop"), and parallel processing (using "par"), among others.

This capability is essential for adequately reflecting the flow of operations within a system, accommodating scenarios where decisions need to be made, or actions must occur in loops. Thus, the introduction of added control logic effectively enhances the expressiveness of the sequence diagram, making it possible to represent the intricate dynamics of system interactions over time with precision. In this way, combined fragments empower modelers to convey the intended behavior of a system under various circumstances.