Hi Naren
If look in IDES, it is or the Return Item Category is activated with "Availability Check Mark"
I have attached the screen shots with this of std Return delivery Item Category & ZREN category of my project , please find the attachment
Also I have attached attached the Screen Shot of,
Determine Procedure For Each Delivery Item Category
where for Std Return Item Category it is Marked with Check Availability& I marked ZREN item category as Do Not Check availability
I hope this will make your doubts clear
Regards
Avinash