An LDAP objectClass of type 'AUXILIARY'. From FC4512:
Auxiliary object classes are used to augment the characteristics of entries. They are commonly used to augment the sets of attributes required and allowed to be present in an entry. They can be used to describe entries or classes of entries.
Auxiliary object classes cannot subclass structural object classes.
An entry can belong to any subset of the set of auxiliary object classes allowed by the DIT content rule associated with the structural object class of the entry. If no DIT content rule is associated with the structural object class of the entry, the entry cannot belong to any auxiliary object class.
The set of auxiliary object classes that an entry belongs to can change over time.