Customer Exits 
Important Note: Since Release 4.6A, SAP has provided a new enhancement technique in the form of 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 make enhancements of the SAP standard more uniform, 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 through the appropriate pushbutton.
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 functions to standard applications.
Enhancements
of the SAP Standard with Customer Exits
Searching for
Applications with Exits
Activating and
Deactivating Projects
Creating
Customer-Specific Menus
Creating
Customer-Specific Subscreens
Creating
Customer-Specific Function Modules