Originally Posted by FrankTalk

"/f bc=" restricts the reservation into a specific bucket but that might or might not correspond with the fare bucket being used (like YY fares booking into lower buckets all the time, shorthauls booking into different bucket than the longhaul, other participating airlines using different fare buckets, etc).

The full syntax for ".." is:

"/f [airline].[origin+destination].[fare basis]"

eg "/f UA.SFO+ORD.FUA2F" looks for UA SFO-ORD fares (only!) that are called "FUA2F". You can wildcard fares, eg "F-" means any fare that begins with "F" ("FUA2F", "FUA" and "F").