This information explains why you would want to use an Enterprise Identity Mapping (EIM) domain controller.
An EIM domain controller is a Lightweight Directory Access Protocol (LDAP) server that is configured to manage one or more EIM domains. An EIM domain is an LDAP directory that consists of all the EIM identifiers, EIM associations, and user registries that are defined in that domain. Systems (EIM clients) participate in the EIM domain by using the domain data for EIM lookup operations.
Currently, you can configure the IBM® Directory Server on some IBM 
platforms to act as an EIM
domain controller. Any system that supports the EIM APIs can participate as
a client in the domain. These client systems use EIM APIs to contact an EIM
domain controller to perform EIM lookup operations.
The location of the EIM client determines whether the EIM domain controller
is a local or remote system. The domain controller is local if the
EIM client is running on the same system as the domain controller. The domain
controller is remote if the EIM client is running on a separate system
from the domain controller.
Note: If you plan to configure a directory server on a remote system, the directory server must provide EIM support. EIM requires that the domain controller be hosted by a directory server that supports Lightweight Directory Access Protocol (LDAP) Version 3. Additionally, the directory server product must be configured to accept the EIM schema. The IBM Directory Server for iSeries™ and the IBM Directory Server V5.1 provide this support.