The application program provides the data that you want to insert in the form. Therefore, you already know the types of the corresponding variables.
You define the form interface to pass data to the form. You use the interface to define import and export parameters, tables, and exceptions. By default, the interface contains a number of SAP Smart Forms standard parameters, which you can use to control form output from within the application program.
Features
The form interface of a form has the same structure as the interface of a function module in the system, except that the form interface does not support any changing parameters. Standard parameters are optional.
The semantics of the valid parameter types match the semantics of a function module. See also Getting Information about Interface Parameters,
Overview of Function Modules (in ABAP Workbench Tools)
Subprograms and Function Modules (from the ABAP: Programming (BC-ABA) documentation)
You defined the form interface. You can use any parameters specified there as fields in the form.