Nieldsdc suggests: "I think they just want to prevent you from flying the shortest sector in Y". In fact, it would make more sense to me if they wanted to prevent you from purchasing the longest segment, because the price of a fixed segment is idependent of distance.

Dave Noble makes the point: "How can you have an extra sector before the normal sectors?". My answer to that is that if you purchase an AONEx or a DONEx, and the rules say that you can purchase an extra segment in L class, then why shouldn't you be able to position that L class segment wherever one likes in the sequence of 'free' segments to maximize the chance of being able to upgrade? In the North American part of my itinerary I have six AA flights and one CX flight (the last of the seven). I can't upgrade the CX flight but probably can upgrade some of all of the AA flights - hence I want to make one of the first six flights the 'purchased' one.