Teams have many players. They don't all play each game. These are characteristics that can be assigned to players for each game.
NOTE: these characteristics can only be added to players in a specific game, not to all team members.
Hover over Competitions and select Team Setup.
Find your team and click on the name of the team.
Follow the steps to add players to your team. If you are unsure of these steps, please see How do I add members to a fixture?
Next to the names of the players you have selected are some characteristics boxes. You can give your players numbers and positions, mark them as starting players, captain or fill-in.
When you have finished, click Save.