Fare Class Availability
 

To get from point A to point C, there are no direct flights. Must connect at B.

From A to B fare class S is available on flight 1 (S9)
From B to C fare class S is available on flight 2 (S9)

From A to C, fare class S is not available on flight 1 connecting to flight 2 (S0)

All the rules met, S fare is published for all the cities, B is valid connecting city etc.

I thought that is S was available for AtoB and BtoC, S would be available to go from A to C connecting through B. Obviously thought wrong, and wondering if someone could explain this....

Also appears to me that Flexible Date Search prices using fare classes available on individual legs as opposed to the trip, and will show a low price based upon that availability; but, when you click through to select the flight, if that class isn't available in the "hidden" A-to-C via B bucket, the website wants to reprice higher. (Also appears that web support, upon verifying the website prices it out a lower price, will honor the lower fare, but will book it into a higher fare bucket. Can work out very nicely if you get charged the 'W' fare, but get ticketed in 'B' on an int'l itinerary which then allows you to upgrade with a piddly amount of miles......)

This is a case of "married segments" - happens all of the time, even on award tickets. If UA has more competition from other carriers from A to B and from B to C, it might make seats available in cheaper fare buckets. In the case of A to C where UA might face less competition, it may restrict the fare buckets and can therefore charge more for the flights.

Fare Class Availability
 

I encounteed the OP's situation yesterday while trying to book SMF-PHL in L. I have found that I can often get around this by asking for a multi city itinerary on .bomb. Even though SMF-PHL was showing L0 alll day, I was able to book SMF-DEN then DEN-PHL in L. It helps to be as specific as possible in your search request. Let's say you know in advance that your desired fare bucket is available from A to B and then from B to C for a particular date/time/flight. Rather than pricing as two individual segments A-B plus B-C, united.com will generally give you the A-C price. This happens even if A-C is showing zero availability in your fare bucket, assuming you're connecting through a valid point for less than four hours (domestic) or 24 hours (int'l).

I just tested my normal travel route (SAN-ORF) and found major pricing differences between round-trip ($480) and multi-city ($340) on .bomb. Thank you for asking the question... I have learned something valuable today.

The whole business of fares is incredibly complex. In fact, doctoral theses have been written on the subject (and most textbooks are just about as comprehensible). In a nutshell, though, fares are based on beginning and ending points, not on the individual segments involved.

The challenge for the airlines is to maximize revenue. This involves estimating the demand for tickets in each market served, and how best to make the limited number of seats available. For the A-B-C example, selling individual segments A-B and B-C will certainly bring in more revenue than the same fare bucket A-C. Complicating the issue is that there are also flights D-B, E-B, F-B, and G-B, along with B-T, B-U, B-V, B-W, B-X, B-Y, and B-Z. Trust me, unless you're a theoretical mathematician you don't want to see the algorithms used for determining fare availability!