Different Types of Selection Screens in sap abap programming

What is:
1. at selection-screen on field
2. at selection-screen output
3. at selection-screen block
4. at selection-screen on value-request
5. at selection-screen on help-request and their difference?

For knowing Selection-screens:

First you must have right understanding of Events.
- Events are introduced by Event Keyword. They end when again next processs begins.

Selection-screens are special screen defined in ABAP.
- This ABAP at run time only controls the flow logic of Selection-screens. The PBO and PAI triggers the num of. selection-screens.

The basic form of the selection screen events is the AT SELECTION-SCREEN event. This event occurs after the runtime environment has passed all input data from the selection screen to the ABAP program. The other selection screen events allow programmers to modify the selection screen before it is sent and specifically check user input.

At Selection-screen OUTPUT is trigerred in PBO of selection-screen.
- This allows you to modify the Selection-screen, before it is displayed.

At Selection-screen On Field is triggered in PAI of selection-screens.
- The input fields can b checked,in the corresponding event block. If an error message occurs within this event block, the corresponding field is made ready for input again on the selection screen.

At Selection-screen On Block is trigerred in PAI event.
- You define a block by enclosing the declarations of the elements in the block between the statements SELECTION-SCREEN BEGIN OF BLOCK block - END OF BLOCK block. You can use this event block to check the consistency of the input fields in the block.

At Selection-screen On value request.
- This event is trigerred for F4 help.

At Selection-screen On help request .
- This event is triggered when the user clicks F1 for help on fileds.