Originally Posted by KIR@GCM
The question poised in the thread title is a very good one. International fare construction can be quite interesting because even if the booking code is "S", that does not necessarily mean that the non-AA segment will also book into "S".
It seems quite variable what class a given fare code will book into when including airlines other than the one quoting the fare. And the detailed fare rules in Expert Flyer tell a lot, but not this particular piece of information.
I have an AA BEE fare to Vienna next month, and that books B class to FRA and then S and Q for FRA-VIE and VIE-FRA respectively on Lufthansa.