Oh...for team training, just see which stats needs improving most, or the stat you feel the players need.
Obvious choices are improving speed for Wingers, shooting for Strikers, defence for Defenders.
But sometimes, you may wanna improve other stats like body balance for wingers if they get knocked about by the opposition's defence too much or improve their passing abilities if their crosses are poor.
Or your own defence may be struggling to keep up with pace of the opposition's strikers, then train them with speed.
Just do what you think is best for your players and train stats according to your own experience.
Alternatively, you can take the easy option of doing auto-train which is more or less the extra stats you gain from a standard league match for every player.