When to Create a Subtype Relationship

You should create a subtype relationship when:

Entities share a common set of attributes. This was the case in our previous examples.
Entities share a common set of relationships. This has not been explored but, referring back to the account structure, you can, as needed, collect any common relationships that the subtype entities had into a single relationship from the generic parent. For example, if each account type is related to many CUSTOMERs, you can include a single relationship at the ACCOUNT level, and eliminate the separate relationships from the individual subtype entities.
Business model demands that the subtype entities should be exposed in a model (usually for communication or understanding purposes) even if the subtype entities have no attributes that are different, and even if they participate in no relationships distinct from other subtype entities. Remember that one of the major purposes of a model is to assist in communication of information structures, and if showing subtype entities assists communication, then show them.