Talent determines how many stars a player can achieve for any position. The position rating is basically a skill showing how good a player is at that position. Other skills do NOT factor into this, so you can have a player with 1 passing, but still have 4 or 5 star QB rating.
4.5 and 5 star talent players can achieve a 5 star position rating.
3.5 and 4 star talent players can achieve a 4.5 star position rating max.
2.5 and 3 star talent players can achieve a 4 star position rating max.
1.5 and 2 star talent players can achieve a 3.5 star position rating max.
.5 and 1 star talent players can achieve a 3 star position rating max.

While the players other skills are important to how he performs, position rating appears to be one of the most important, if not the most important, stat a player can have.

PHI 21 wrote: While the players other skills are important to how he performs, position rating appears to be one of the most important, if not the most important, stat a player can have.

This is key knowledge