“Special” value sets use user-exits to execute the code written to validate the value entered in a field (parameter window). As the validation code would be executed by C program (user-exits), it needs to be written abiding certain syntactical rules. Also we need to specify the event at which the special validation routine should fire. Valid events are Edit, Validate and Load.
Note: We would mainly explore around the Validate event
To validate whether entered value is as per validation or not.
It validate when user enters value which is not fulfilling the condition