Add new attributes when the existing attributes do not support all of the information you need to store in a directory entry. Try to use standard attributes whenever possible. Search the attributes that already exist in the default directory schema and use those attributes in association with a new object class.
For example, you might find that you want to store more information on a person entry than the person, organizationalPerson, or inetOrgPerson object classes support. If you want to store birth dates in your directory, no attribute exists within the standard Directory Server schema. You can create a new attribute called dateOfBirth. Allow this attribute to be used on entries that represent people by defining a new auxiliary class that allows this attribute.