Customer Exits 
Important Note: As of Release 4.6A SAP provides a new enhancement technique, the Business Add-Ins. Among others, this enhancement technique has the advantage of being based on a multi-level system landscape (SAP, country versions, IS solutions, partner, customer, and so on) instead of a two-level landscape (SAP, customer) as with the customer exits. You can create definitions and implementations of business add-ins at any level of the system landscape.
To unify enhancements of the SAP Standard you can migrate customer exits to business add-ins.
If you used transaction SMOD to migrate customer exits, the enhancement projects of the customer that belong to the customer exits must be migrated to implementations of business add-ins, in order to preserve the functionality of the enhancement. After the upgrade, start migration in transaction SPAU via a button.
If no migration was executed for customer exits created prior to Release 4.6A, you can still use the technique described below to add your own functionality to standard applications.
Enhancements to the SAP Standard with Customer Exits
Locating Applications that Have Exits
Activating and Deactivating a Project
Creating Customer-Specific Menus
Creating Customer-Specific Subscreens
Creating Customer-Specific Function Modules