When developing corporate-level business applications (Enterprise 
Scale) customers often requires implementing support for extensibility 
of the application object model not modifying the system source code. 
Use of extensible domain model allows for development of new 
functionality without additional effort and overheads:

1) the application will be used for a more lengthy period
2) the system workflow can be modified over time when changing external 
factors
3) “setting” the application to fit specifics of an enterprise where it 
has been deployed.
The most simple and cost effective way to achieve the required 
functionality would be implementing extensible business entities in an 
application with the support of custom fields.

For more details go to http://techzone.enterra-inc.com/?cat=6