extension property

An extension property is an attribute extensions defined in an extension schema. It is based on a property definition (usually attribute, reference or relationship) defined in an extension schema, which can be added as attribute extension to any (extendable) object instance. In case of automatic extension, extension properties are considered as potential properties of any (extendable) object type. In case of managed extension

In contrast to attribute extensions, which are transient extensions, extension properties are, typically, persistent properties stored with the instance.