Originally Posted by
Calchas
On most US/AA domestic flights under US/AA codes, business fares book into the A booking code, not J/C/D/R/I. (On the three-class transcons, currently operating between JFK-LAX and JFK-SFO this does not happen.)
You sort of hinted at the explanation already. Nearly all U.S. AA domestic service is two class, so J will book into F. The three class transcons are, well, three class; so J books into J.