Originally Posted by xliioper

Correct. Domestic FC fares are almost all dual-inventory fares (except for full J) which start with a Main cabin coach class in fare basis code - Y/B/M/H/Q/K/L/U/T/X/V. Even though they start with Main cabin class codes, they are still fully First Class fares (the second to last character in the fare basis code will always be an 'F' to designate that). There are none that start with the letter 'E' which is reserved for Basic Economy fares. In order to book a particular FC fare, there must be both inventory in the Main class that starts the fare basis code, and the FC booking class of the fare (J/C/D/I/Z). Hence the name "dual-inventory".

BE and C+ fares are also dual-inventory fares that start with Main cabin classes in their fare basis codes. BE fares always book into 'E' booking class, while C+ fares now have two booking classes (S and W). PS fares are a mixed bag with domestic, Asia, and South America PS fares being dual-inventory, while EU PS fares are not (first letter of fare basis codes will be the same as the PS booking class -- P/A/G).