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Attribute names in SAML, and by extension Shibboleth, are generally expressed as URIs. Section 8.2 of SAML 2.0 Profiles recommends that all LDAP attributes be expressed as URIs through transformation of the OID identifier, e.g. urn:oid:1.3.6.1.4.1.5923.1.1.1.5. Attributes may also be named through other URN namespaces, or by using URLs, which could be resolved to yield a controlled vocabulary, synonyms, and other information. It's very important that proper attribute naming etiquette be followed and that you don't define attributes in namespaces you don't control.
Proliferation in attributes and misappropriation of attributes are two countervailing forces that need to be balanced as you decide which attributes to use. A large number of attributes has already been defined and named (e.g. eduPerson and x.520), and in any situation where a pre-existing attribute meets your needs, it should be used. However, controlled vocabularies and specific definitions must be followed. Use your best judgment in deciding whether to create a new attribute definition.
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