One position means that you get maximum efficiency when you train a max of 3 players for 48 minutes in one position a week. Certain trainings are one position while others are two positions. For example, Pressure is one position training for PG, Inside Shot is one position for C, but Rebounding is two position for C/PF. You can tell which is which when you switch trainings in the Training menu. Each training will default to it's most efficient version/position when you select it. If my memory serves, choosing to change position may also give you a percentage as how much training efficiency is lost by training more or different positions than the training is intended for.