This is an interesting thread, but BBs didnt said that it is true.
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Position is only formulation of proportion of skills and has no other purpose.
It is certainly not true because it doesn't count with sublevels and they counts towards salary for sure. Whether position have some impact towards salary is another thing but I don't really care because no matter why salary is calculated the same players will have the same salary and the same position as well so technically position doesn't change anything - you can't get any advantage by the position of a player (in comparison to similar player with different position) because as soon as he would get the same (or more similar) skills his position would be changed too. And if some player of similar skillset is cheaper it is just because skills he has are a bit cheaper (considering equal sublevels) not because he is considered to be best at different spot.
That's why it would be nonsense if positions would have any impact towards cap. Since position can instantly switching every training update it would mean potential cap would change too and that would be dumb. Most importantly we don't know for sure how the cap works. But the cap could be triggered by certain overall level of (all) skills or by crossing a certain level of salary. Ii believe the first is the case and actual salary and position act only as indicators something like fan survey for attendance; not causing it but indicating why something is happening.