What is a constraint in modeling?

A constraint in modeling is recognized as a Boolean expression that is typically contained within curly braces. This expression serves to impose limitations or rules on the elements of a model, defining what can or cannot occur within that context. Constraints are vital for maintaining the integrity and validity of models by providing specific conditions that must be satisfied.

For instance, in UML (Unified Modeling Language), constraints can specify conditions for attributes, operations, or relationships, thus allowing for a more precise and robust definition of behaviors and properties within modeled systems. These constraints enable modelers to articulate business rules or system requirements clearly and unambiguously.

In contrast, the other options represent different concepts within modeling. A solid line indicating a synchronous message is related to communication between components in sequence diagrams, a textual explanation of a message refers to documentation or annotations accompanying messages, and a method signature in a lifeline pertains to defining operations within a specified context. Each of these elements serves different purposes in modeling and does not encapsulate the notion of a constraint.