As we already know players have different skill sets - shooting, passing, crossing, etc. and these skill sets are measured with a 0-99 rating system. Assets can be used as a booster for each of these skills. When you connect a certain asset to a football player, it boosts the player's rating for the specific skill by some percent. For example, if a player has 50 rating for shooting skill, using a shooting asset with 20% boost will increase the player's shooting skill to 60.
Only 5 assets can be attached to 1 player. These 3 assets can be used for boosting different skills (f.e. Shooting, passing, heading) or one specific skill (f.e. passing).