Fatigue settings control substitutions based on the depth chart. If he's lined up as an RB, the backs setting will control. If he's lined up as a WR, the receivers setting will control. If he's lined up as both, it's based on the fatigue of the players in the depth chart at both positions compared to each setting, to determine where he's needed for a particular play. If he's the top of the depth chart at more than one position, it's random until his fatigue reaches a setting for one position. The position he suits up at can affect weight and thus indirectly affect fatigue, but it doesn't directly interact with the positional fatigue settings.
