I'm pricing up some flights I may need to take next Feb from IAD to DEN (via DFW) in F. If I search for flights on AA.com using the "Price/schedule" option I'm offered fare code "F" but if I seach using the "schedule" option (and selecting Flexible First) I get offered "P" flights which are priced approx 20% lower. Is there any real difference / restrictions between F and P type fares - there does not seem to be anything highlighted during the booking phase.