Customer Exits 
Customer Exits
This section explains how you can use customer exits to add your own functionality to standard R/3 applications.
Enhancements to the SAP Standard with Customer ExitsTypes of Exits
Locating Applications that Have Exits
Creating an Add-on Project Activating and Deactivating a Project Transporting Add-on Projects Creating Customer-specific Menus Creating Customer-specific SubscreensCreating Customer-specific Function Modules
More information on this subject can be found in the online help of the Project management screen for SAP enhancements.