HI Anshu,
Here is all details regarding I_steps, ignore first reply...
I_STEP 0 : This is a step where you can do authorization checks..
The enhancement is not called from the variable screen. The call can originate from the authorization check or from the monitor.
Scenario: If any user can able to execute query or not, this can decde in this step.
I_STEP 1:let us consider scenario, there is query which is on filter as 'Calday'.. and whenever we execute query then it should ask user date to be entered. AND on date popup there should be calday as today's day populated automatically..
then for this case you have to implement code as "variable = sy-datum" in step 1.
i_STEP 2: If you want user to enter manual inputs then you can put it in this step.
Scenario: Let us consider ex, where on execution of query , user can able to enter company codes for which he wants to see output..
so this you can do in i _step 1..
like select * from table into internal_table where Company_Code = User entered CC.
I_STEP 3: In this step you can validate the entered values by user in i_step 2..
Scenario: in continuation to i_step 2, suppose instead of company code user enter some wrong value (ex his name 
in stead of company code), then select query will give dump..
but you handle this in i step3 , by validating user entered values..
So in short here is complete picture for i_steps ..
1) suppose u execute query then it will check your authorization in step 0.
2) suppose after execution you want some default values in pop up, then you can do this in i step 1.
3) after popup u can enter your desired inputs in popup, in i_step 2.
4) Whatever, you entered in step 2 will check/validate in i step 3 .
Regards,
Hitesh