What separates regions within a combined fragment?

In UML (Unified Modeling Language), a combined fragment is a composite structure that represents a sequence of interactions. Within a combined fragment, regions are separated to define the different parts of the interaction that are controlled by specific operators, such as alternative, parallel, and more. The separation of these regions is visually represented by dashed lines.

Dashed lines are used to indicate that the areas they bound can contain distinct execution paths or conditions. This is important for understanding how various scenarios in interaction diagrams can interact or diverge based on the specified operators. Each region can encapsulate different messages and their corresponding lifelines, thus making the overall structure clear and manageable for complex sequences.

In contrast, other types of lines may represent different concepts or frameworks in UML, such as solid lines denoting lifelines or objects, or dotted lines may indicate dependencies or associations rather than the specific function of demarcating regions in a combined fragment. This focus on the specific use of dashed lines for separating regions underscores the importance of clarity in both communication and design when modeling interactions.