A Constraint element identifies a constraint on other elements, and can be connected to other elements of any type. The Constraint element icon is available in any Enterprise Architect diagram, through the 'Common' pages of the Toolbox.
This element is a means of documenting the fact that there are constraints on related elements; it has no impact on the other elements. However, a Constraint is a named element, listed in the Browser window, and able to be copied and re-used where appropriate. You define the types of constraint in the project reference data, apply them to the element in the element 'Properties' dialog, and manage them through the Responsibilities window.