The game manual says,
Best Position: As with the DMI, this is a very rough guideline, and some would even say a meaningless one. You should play your players at the positions that make the most sense to the team, regardless of whether it is his listed position.
From the second drop-down menu, you can choose which player positions to train. Players will only get better with a combination of out-of-game training and in-game experience, so you can have the coach choose to train one or two key players per week, who will then show a rapid improvement, or to train three or more positions with reduced effectiveness.
For a player to improve a skill, the position in the training plan must match the position they play on the court.