What are the differences between user, screen and field exits?

Let me answer this question  for you:
 
The basic rule in SAP as far as ABAP workbench developments are concerned is "Do not modify the source code of the program"
 
Since needs and requirements keep on changing from client to client and for various business scenarios, there is a need to customise the programs as per client requirement.
 
For this purpose, SAP has provided numerous list of user-exits also known as enhancements, which is nothing but a program which will deal with the client additional requirement, without modifying the source code of the program. These exits are already provided by SAP. You can see the list of user-exits provided by using the transaction code SMOD.  If you cannot find an user exit which match with client requirements, then you can tell SAP to create a new user exit.
 
On the other hand, field exit is nothing but where you write a small program, to control the properties of an existing field in a particular screen.  Hence creation of a field exit is controlled by Program name, screen no. and the field name.

To give you an example, if you want to prevent a particular payment method to be executed via transaction FBZ5 or F-58, such that if the user enter a particular payment method while executing the transaction FBZ5 or F-58, the system should restrict the user from proceeding further, u can write a field exit for field RZAWE (payment method).

The program which controls the creation of field exit is RSMODORF
 
As far as screen exit is concerned, as per the client requirement if the user wants a separate screen to appear on execution of a particular task, which deviates from the standard sap provided flow of transactions, you write a screen exit.  In such case you first prepare the screen using the screen painter and define it as to after which sap screen you want this screen to appear for processing the transaction further.

Tips by : Rakesh Jain

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