I could have sworn I read an article on this a few years ago, but cannot find it. I'm just curious.

Say I book a ticket from AA and the reservation code is ABCDEF.

Do those letters mean anything? Is there any rhyme or reason to which letters the PNR is assigned, or is it totally random? Any link to geography or POS or type of ticket?

Is this true for both Sabre and Amadeus?