Training focus
On the Club > Training tab each player focuses on ONE attribute.
Stamina can be the focus too, but it trains at half rate.
Focus is a steering bonus: it decides which attribute matures first.
It does not change how much total growth a career holds; that is player development.
"Auto pick for everyone" sets every focus to the attribute with the most room to grow for that role, in both modes, and re-picks a focus you set by hand too.
A player nothing can help keeps their focus, your player is skipped, and one line tells you how many changed.