Dear Kalyana,
As far as your requirements are concerned, you should use create Purchase Requisition = '1' during MRP run.
'Purchase Requisition in opening period' means that PR's will be created in the 'opening period'. This 'opening period' is derived from material master scheduling margin key. Outside the 'opening period', system will generate planned orders for externally procured materials.
These planned orders are subjected to be converted to PR's, not to production orders. In COHV mass processing, they are hence not selected.
System is updating the fixed indicator, because you are performing manual changes on MRP generated 'planned receipts'.
Best Regards,
Rajen