Hi Phani
As mentioned by Paul, that is the precise reason for reserve.
In standard, SAP Business Workplace, when you open a work item/Decision Item, the system first reserves the WI so that if it is assigned to multiple agents, it is pulled out from their inbox (as you are now processing it). If you close the screen, without acting on it, it remians reserved with you (other common agents cannot see it). SAP Inbox has given explicit RESERVE and UNRESERVE buttons for that. If you open an item, it gets reserved, however you do not want to act right away, you can close the screen and unreserve it explicitly so that other agent (if any) can again see the item in their inbox.
Regards,
Modak