When you add pennants to a particular team, any players on your roster belonging to that MLB team life will get a boost for Season Mode games.  You may choose to distribute your buffs evenly to all teams, or just a few favorites.  

 

Pennants can only be earned by playing Season Mode games. Get Pennants from end-of-game rewards and Milestone rewards. 

 

 You may choose to add pennants to as few or many teams as you wish.   When you boost a particular team here, any players belonging to that team will show a boost in your roster to the lower left of the player.  

Pennant buffs are based on the real-life MLB team in which the player started the current season.  (Trades that happen during the actual MLB season do not affect pennant buffs.)

 

Pennant buffs only apply to Season Mode.