Purpose of constraint is to narrow (to constrain) actual values that get written into the per-entity instantiated Attribute. For example, you might want to contrain named Attribute values to predefined set of numbers or strings, expression-based ranges, or utilize already instantiated Connection to fetch the predefined data set within external catalog (database).

[SerializableAttribute]
public class Constraint : Feature

<SerializableAttribute>
Public Class Constraint
	Inherits Feature

[SerializableAttribute]
public ref class Constraint : public Feature

[<SerializableAttribute>]
type Constraint =  
    class
        inherit Feature
    end