LDAP Schema is a convenient packaging unit for containing broadly similar objectClasses and attributes.  An LDAP schema must contain at least one object class.  An attribute added to one schema can also be used by an object class of another schema.  It is necessary in LDAP schema that every attribute or objectclass, including its superior objectclass or attribute, used in an LDAP implementation must be defined in a schema, and that schema must be known to the LDAP server.