User Exits and User-Defined Form Routines 
Use
You can use the Interface Toolbox to call your customer form routines during the conversion. This enables you to implement complicated file layouts.
If a user-defined form routine is called up, the Interface Toolbox transfers parameters to the user-defined form routine. The form routine transfers a return value to the Toolbox.
If you want to implement a user-defined form routine using the user exit, you must specify which form routine is used and in which program this form routine can be found.
Prerequisites
The user-defined form routine must meet two criteria:

If you use a user-defined form routine, you can choose to set the first 15 parameters. These parameters are used as input parameters. If a parameter does not have an entry, then the Interface Toolbox uses SPACE.
You can use the following types of input parameters: