These are the various objects that can be placed on the screen and help in proper functioning of the screen. Following is the list of screen elements

Text field
A field which is output only, used for displaying hard coded static text on the screen. This cannot be modified during run time. If the text contains several words, they should be separated

Input/Output field
A field used by the user for data-entry. This is also called a template. Use underscore characters in the text field to define its size. The maximum length can be 255 characters.

It is the code that processes a screen in SAP. The various event blocks of flow logic are as follows
Process Before Output PBO
Code that is executed before the current screen is displayed
Process After Input PAI
Code to be executed when the user does any interaction with the screen. At the end of PAI the system goes to the PBO of the next or the current screen
Process on Help-Request POH & Process on Value-Request POV

R/3 provides three "customization spots" that allow you to enhance FI/CO features without modifying the standard code. Although often collectively referred to as "user exits," two of the three have different names. SAP Enhancements are used to expand the standard functionality within SAP. Enhancements use function modules and are called from the standard SAP code. Each module in the system has a set of delivered enhancements that help companies expand the standard functionality where they need it. Enhancements were new in release 3.0.

Make sure the application is active for Business Transaction Events.
Copy the sample interface function module into a "Z" function module.
Enter the ABAP code into the source code section of the new "Z" function module. You may choose to create a "Z" program to enter the code into and then insert the "Z" program into your function module source code.
Activate the function module.
Assign the function module to the event, country and application.

Whenever user does any interaction with the screens, a field is required to trap the action that the user is trying to perform. To do this, define 4 character variable by the name v_okcode_screennumber.
Assign function codes to the various buttons on the screen that the user is likely to press.
These function codes are then transferred via the variable to the flow logic of the screen, to decide the path to be taken by the flow of the transaction.