Central User Administration 
Using Central User Administration, you can maintain user mater records centrally in one system. Changes to the information are then automatically distributed to the child systems. This means that you have an overview in the central system of all user data in the entire system landscape.
Distribution of the data is based on a
functioning Application Link
Enabling landscape (ALE Landscape). In
this way, data can be exchanged in a controlled manner and is kept consistent.
An ALE System
Group is used by the Central User Administration to distribute user data
between a central system and child systems linked by ALE. You should therefore
familiarize yourself with basic information about the 
ALE Integration
Technology.
Central User Administration data is distributed asynchronously between the application systems in an ALE environment. This ensures that it still reaches the target system even if it was unreachable when the data was sent.
One system in the Central User Administration ALE environment is defined as the central system. The central system is linked with every child system in both directions. The child systems are not linked to each other , with the exception of the central system, which is itself a child system, from the point of view of Central User Administration.
Use the most up-to-date system in your system landscape as your central system (if possible with a release status of 4.6C or higher). In this way, the newest functions in CUA are available to you.
See also:
If you have this document as an excerpt in the form of a .pdf file, you can obtain the complete documentation in the SAP Library under the path SAP NetWeaver ® Security ® Identity Management ® Users and Roles ® Central User Administration.
· SAP Tutor Central User Administration under service.sap.com/security ® Education and Workshops
· Authorization Made Easy Guide Book – Chapters 10 and 11 (www.saplabs.com/simple, www.amazon.com, or www.fatbrain.com/sap)
·
ALE: Implementation
and Administration