Explanation/Reference:
Dedicated AxC/Directory server: A physical server that manages notification capabilities and the LDAP database and provides communications between application servers and the thirdparty storage server.
This server also stores user properties and name and greeting recordings.
Not all configurations require a dedicated AxC/Directory server because the AxC/Directory role runs on the Avaya-provided message store. You only need a dedicated AxC/Directory server for:
Release 6.2 and earlier Messaging systems with a third-party storage server.

Release 6.3 or later systems with more than one application server, or more than 6000 users using a

third-party storage server, such as Microsoft Exchange.
References: Avaya Aura® Messaging Overview and Specification, Release 6.3.2 (January 2015) , page 20
https://downloads.avaya.com/css/P8/documents/101004642