Actually I did ran MRP with control parameter value '3' for Create Purchase requisition, and still the system creates Pur. req. for STO.
Only when I delete the procurement type 40 and only the "X" remained, the system generated planned orders that can be converted to either production orders or pur. req.
I will look into the quota arrangement to see if can I control the type of document opened according to specific time period.
Many thanks!