Even more or less straightforward itineraries may include flights booked as different airline codes. The mechanics of prorating the base fare of each pricing unit (e.g., outbound vs inbound) by the flight distance of each segment covered by that pricing unit and adding in any YQ/YR are essentially the same and scale from a simple one-way to an RTW. In my experience, multiple ticket reissues (common with my RTWs) tend to make AA's crediting system fall back on distance rather than fare.