If you use consume forecast, NS will take your forecasted demand and deduct any SO's from that amount. Keep in mind, it will only consider SO included in the time fence you set up. As an example, if I have demand in NS at 100 and a SO for 10, MRP will show my demand at 10 + 90 (100-10). If you don't consume forecast, NS will show your anticipated demand at 10 + 100 = 110.