Directory Server is an add-on package based on LDAP version 3 (RFC2251) that allows your Synology NAS to become an account administration center to centralize the account management of all connecting clients, and provides authentication service for them. For more information about LDAP, please see here.
Set up Synology NAS as a Directory Server to provide account authentication service
Back up and restore LDAP database
Manage LDAP users on Directory Server
Manage LDAP groups on Directory Server
Make Directory Server into an identity provider for Google Apps Single Sign-On