From Kolab wiki

Kolab's Directory Service Schema

In the center of a running Kolab solution is always a directory service that holds information about the users, some server configuration values and even external users.

The default Kolab Server/OpenPKG installation comes with an embedded OpenLDAP which is an implementation of a directory service which can be queried by the LDAP protocol. Of course it is possible to use other directory service software, e.g. FDS.

Each directory service has a schema which defines the attributes and more. Kolab uses parts of standard schemas to the maximum extend, but of course needs some attributes of its own. Check the following directory for the schema files of the included OpenLDAP:

/kolab/etc/openldap/schema/

Of special interest to Kolab are

 kolab2.schema
 horde.schema

Kolab itself has been assigned the OID number

 1.3.6.1.4.1.19414

Each subnumber should be assigned and used for one purpose forever. Thus in case of a substantial change, a new subnumber should be used. Please coordinate with the Kolab developers about using one of these OIDs.