An attribute constraint affects the attribute's value in some manner. Has a range or a size limit to the value. Is synchronized immediately, or at the next scheduled interval ,or never. Is hidden or is viewable. Is writeable or is read-only.
Constraints also manage access control to the attribute.
For example, you can constrain an attribute so only users with sufficient rights to the object can add the attribute. Constraints also define the matching rules that are used when comparing two values of the same syntax. A search usually uses the equality, substring or ordering matching rules. The equality matching rule works for attribute types are the same. It checks for equal values of the attribute.