I suggest everyone to read this topic
(208622.1) about the last "Ask BBs", it contains a lot of infos ;-)
About your question, it's very similar to what I asked (you can find it in the linked topic):
Q Biffo:some clarifications about BoA defences: The best player/scorer is guarded M2M. How is "the best" calculated? Before the match or during the match? If before the match, what are the parameters...? Salary? Season average? Others? If during the match I guess that "the best" to guard could change over time. The "fifth player" is used to defend M2M. Who is the "fifth man"? The best (outside/inside) defender? Or just the player set in the same position as the opposite "best player"? (eg. the best opposite team's player is a PF... your M2M defender is your PF no matter what?). Is the "fifth man" fixed or changes over time? Why didn't you designed the system to allow managers (us) to decide which man (or position) to guard and which man to use in M2M defence?
A Marin:While I can't disclose all the details, I can give you a general idea of how the BaO defense works. Before the game starts, both the best offense and defense players are calculated based on their skills, with the offense/defense type in mind. You cannot "set" those players, but you can probably guess who they would be, using some rational thinking and common sense. It was intentionally coded that way. Those player roles remain the same even after substitutions occur, so there are no changes during matches. I may have even said too much already :P