According to the UML specification, a Constraint is an assertion that indicates a restriction that must be satisfied by any valid realization of the model containing the Constraint. A Constraint is attached to a set of constrainedElements, and it represents additional semantic information about those Elements.
A Constraint is evaluated by evaluating its specification. If the specification evaluates to true, then the Constraint is satisfied at that time. If the specification evaluates to false, then the Constraint is not satisfied, and the realization of the model in which the evaluation occurs is not valid.
Visualization
The constraint is normally depicted on a diagram with the constraint string is placed in a note symbol and attached to each of the symbols for the constrainedElements by dashed lines.
How to create a Constraint and associate it with a Model Element
The Constraint is a definition in UML 2.5 and SysML support. You can create a Constraint and assign it to a Model Element in the Element UML tab of the Model Element’s definition.
You can also assign the Constraint to the definition visually through the Constraint to ConstrainedElement relationship line on a UML diagram.
