A constraint is a validity conditions for an object or property instances. Typically, constraints are defined for properties or structures in data model definitions. Constraints are defined as conditions (expressions), which must be defined as valid expression in the structure context, which is the structure for a structure constraint and the structure a property belongs to for a property constraint.
Constraints are considered as weak consistent, i.e. constraints will be checked, whenever the data in a property a structure instance changes. Since constraints may refer to far properties, consistency cannot be guaranteed, when far properties change.