I just came across this today and I have a screenshot to help explain it:
The player I am moving can play as a DC or DMC. Those positions are highlighted, but the full back positions and the MC position is highlighted in yellow. And the other positions are dark. So I am just guessing here but I think nordeus has made it so depending on a players position, he is able to play in certain positions without having a big morale or possession penalty. Anyone else have some input?