Just as an example, that flight's full inventory right now is
"Available Cabins: J9, JN9, C9, D9, Z9, ZN9, P9, PN9, R0, RN0, IN0, I0, Y9, YN9, B9, M9, E9, U9, H9, HN9, Q9, V9, W9, S9, T9, L9, K9, G0, N0, XN9, X9"

So from an inventory perspective, it's pretty much "wide open" (except for a few upgrade and award buckets and the deep-discount G & N buckets).

But even though the flight has "K9", if I do a one-way search EWR-CDG, the cheapest fare UA will sell me on that flight is a B fare. That is, there are no valid UA fares in the cheaper buckets M, E, U, H, Q, V, W, S, T, L, or K for the kind of itinerary I want to buy — a one-way trip EWR-CDG with purchase on 10/29 for travel on 12/5.

Now, if I do a round-trip search for, say, 12/5 to 12/12, I am offered a K fare, fare basis KLXNC8S. So there is a valid K fare posted for round-trip travel between EWR and CDG for original departure on 12/5, return on 12/12, purchase on 10/29. But note that I only got this by meeting the rules of a published fare.

If you'd like to find deeply-discounted "P fares", try (1) first finding the fares posted and (2) *then* finding fare class availability.

Here's a quick snapshot from ExpertFlyer of all round-trip business-class fares posted by UA for travel EWR to CDG:



As you can see, there's a deeply discounted P fare ($933 round-trip base fare, not including taxes and surcharges) but it's only valid for Thanksgiving (original departure 11/24 thru 11/29). And there's another deeply discounted $1400 P fare valid for Christmas travel (original departure 12/14 to 1/3, minimum stay 3 days, 28-day advance purchase requirement). The next cheapest P fare is not a great bargain ($2440 base fare) and carries a 50-day advance purchase requirement and 8-day minimum stay.

So to recap— if you want to find a cheap trip and you think you can do better than Kayak/ITA/Hipmunk/Expedia/etc., first find the fare basis, *then* look for inventory that matches the fare you want.

A bit tough to find fare data for free. Expertflyer and KVS offer fare information on a paid basis. Travelocity will monitor fares for you but I've never found their tool useful ( http://travel.travelocity.com/mystuf...FareWatcher.do ). There used to be a great "Flyertalk tool" published by Farecompare, but they have discontinued it, either because they're lazy or because of regulatory reasons regarding advertising of flight prices. In a perfect world, you would be able to pay ATPCO some money and subscribe to a raw fare data feed, then send yourself an e-mail alert when you found premium-cabin fares priced at below X cents per mile…