I just pushed a new feature to the play cards, to help you understand the player's assignments for each play. There are two new tabs: "Diagram" - which is a large diagram of the play, and "Assignments" - which lists each position and their assignment.
You can get to the play card by clicking on the play name in the game log as before. I have also added a hot spot on the game planning screen, if you click on any of the play diagrams when choosing your offensive/defensive/scout plays, that will open the play card as well.