Hi Shree,
If you create a PurRqs manually at the Product View, it will be firmed regardless of Planning Time Fence. In the Elements tab, PP-Firmed flag is set. If you want to unfirm it double click it to go to Order Processing and uncheck the indicator Output Firmed.
If the order is inside the Planning Time Fence, PP-Firmed flag is set dynamically/automatically and cannot be unchecked from PP/DS side. Technically, everytime you open the Product View or execute a planning heuristic in PP/DS (SAP_PP_002), Function Module /SAPAPO/RRP_RECEIPT_CHECK_FIX is executed to evaluate if the elements are firmed or not. Here, the system checks if Availability Date of the order is within the PTF and, in positive case, sets PP-Firmed flag.
I do not know what impact (if any) the Planning Time Fence has in SNP Planning Book or upon running SNP planning functions.
Regards,
Tiago