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At the top of the eduPersonPrincipalName example above, there is the object property "isClassifiedBy" with the value "Identifier". This is an example of a metadata element meant to categorize attributes across specifications into a defined set of types. This metadata element is called "Attribute Class". Here is the first part of a listing of the currently defined attribute classes: 
A couple examples will clarify the notion of attribute class. Take the example of attributes relating to preferences. 
The Open Social specification (attributes whose prefix is "osoc" contains two attributes in the class "preference". And the LDAP specification (currently RFC4524) contains an attribute "drink" which indicates personal preference as well. Here is the preference attribute, osoc-emails-primary: 
Another example of the attribute class metadata is "role". Several specifications (SCIM, SCHAC, LDAP and X.520) contain attributes meant to carry some definition of a person's role: 
Future Work
Going forward, attributes from additional specifications and standards will be added (including a master set from FICAM and one from the state of Virginia). One open issue is whether the current list of attribute metadata is adequate or whether there would be value in carrying additional metadata elements in a general purpose registry of this sort.