Field exit are use to create programming logic for any data element.It carry out any checks,Conversions or Business related processing of screen fields. Important thing is it can not be debugged.It have its own naming convention FIELD_EXIT_.
In transaction code CMOD choose Text element -> Field exit.
1. Click Field Exit – > Create. Enter Data Element for Field Exit.
2. You are taken to the screen to define the function module/ code that will be called when the field exit is performed. Click Create to create the function for the field.
3. Enter the code in the function and then activate the function module.
After you have created the processing logic for your field exit, you must assign the exit to one or more programs and screens. This assignment tells the system which screens to trigger your function module on. To assign your field exit module to one or more screens,proceed as follows:
1. Select the field exit.
2. Choose Assign prog./screen.
3. Enter the name of the program and the screen number.
For some fields, you might want to trigger a different processing logic on different screens. You can trigger screen-specific logic by allocating a field exit identifier when you make your screen assignments.
A field exit identifier can be any number or letter. The field exit identifier tells the system to trigger the function module that has this identifier in its name.