I think it's a good question. It's what I've been wondering about myself.

What I know: if I use GK training on any other player than GK, they won't gain any skills (because they don't have those), but they will lose condition. Effectiveness is zero.

If the same applies for any drill, that would mean players would gain a percentage in every skill, but only level on the ones they have. So from an attacking drill with 4 def/att skills and 1 GK skill, field players would profit at 0.8 effectiveness, GK's at 0.2.

But that's just speculation. I'm not sure if and how it can be tested.


As for extra positions: they can be gained by any drill, not necessarily the ones related to that position.
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