Constraints are non-view specific elements that can function independently of dimensions. Constraint elements appear as blue dashed lines in all views in which their references are visible; dimensions are view-specific. You can modify and delete constraints independently of dimensions or remove them when you delete dimensions.
In Revit, elements can reference each other in many ways, including the following:
These explicit and implied constraints require that each element knows about the other. That is, one element must reference the other element.
Constraints belong to the worksets of the elements they constrain. In order to add a constraint, the worksets of all the elements that can be moved by the constraint must be editable.