I have (what I hope is) an easy question about the listings on ITN. If this has been posted/answered before I apologize for reposting it. I tried a search but couldn't come up with an answer.

I recently did a search on ITN to check seat availability from DTW to SFO on 6/27. The flight I was interested in came back showing the fare classes with a number next to it (ie. F9 M9 N9 V9 Q9 K9, etc). Does the "9" indicate that there are 9 AVAILABLE/OPEN seats for a particular fare class or that there were 9 ALLOCATED seats for the fare class that may or may not be open? Since I'm always interested in booking the lowest fare possible, at first I was happy to see the K9 come up for the flight I wanted. However, when I called NW to actually book the flight, the representative said that the K fare class was not available on that flight, even though ITN showed it as K9. Could it be that since I was looking at DTW/SFO and then LAX/DTW (instead of a straight roundtrip) that they wouldn't offer me the K fare? If not, am I reading ITN incorrectly?

Any help or clarification is much appreciated. Just when I thought I had most it figured out.....