We can use the CUSTOM library to add or modify functionality without making code changes to the standard form. All of the standard forms have calls to the custom library from specific events that occur at runtime. We can add code to the CUSTOM library to capture the events (such as WHEN-NEW-FORM-INSTANCE, ZOOM, and so on) and then add code to perform additional functionality. If you need to see which events are fired on any particular form you can navigate to Help | Diagnostics | Custom Code | Show Custom Events from the menu and you will see a message appear each time an event is triggered (you may be prompted for the apps password when you do this). The message will show the event that was triggered.
The CUSTOM library is shipped with e-Business Suite and can be found in the $AU_TOP/resource directory. It is recommended that you do not put all of your code in the CUSTOM library. It is a good practice to create a sub library per form and put your code...