New in version 3.1 is the ability to define contact sub-types at CiviCRM>>Administer>>Option Lists>>Contact Types. Sub-types are defined in relation to one of the three built-in contact types and inherit the fields defined in their parent.
Profiles used to create contacts can be set to create contacts of the appropriate sub-type. This setting is implicit in the field selection. Consider a user defined sub-type Applicant of the core type Individual.
The First Name field exists in both the Individual and Applicant types. By choosing First Name from the Applicant type, this profile will be set to create and interact with Applicant contact records.
Fields may only be selected from one sub-type. If you need to change the sub-type, revert all the fields to the parent type or delete the sub-type fields and then redefine them from the new sub-type.