Mapped Superclasses: An entity may inherit from a superclass that provides persistent entity state and mapping information,but which is not itself an entity. Typically, the purpose of such a mapped superclass is to define state and mapping information that is common to multiple entity classes.
Quoting from Spec.2.1.5
Embeddable Classes An entity may use other fine-grained classes to represent entity state. Instances of these classes, unlike entity instances themselves, do not have persistent identity. Instead, they exist only as embedded objects of the entity to which they belong. Such embedded objects belong strictly to their owning entity, and are not sharable across persistent entities.