Trophy Trials is a seasonal event that gives Managers a chance to additionally boost the OVR of their players. 

By competing in rapid matches and earning rewards like Drop Balls, Managers will be able to add +10 increase on player’s Key Attributes. The Manager with the most trophies by the end of the day will top the daily leaderboard and will receive additional Drop Ball rewards.

Drop Ball can be earned either by having a good daily leaderboard standing or by challenging other Managers during the event. One Drop Ball contains rewards such as Limited Tiers (Selected and Random), Tier Credits and Rests. 
Limited Tiers and Tier Credits
Enhance your team’s performance on the pitch by increasing your player’s most important attributes.
Limited Tiers are a seasonal +10 Key Attribute increase that can be applied to all players, regardless of whether they have an unlocked Tier or not. This means that now, you can increase the Key Attributes also for players who currently have less than 180 OVR.
Also, make sure to collect the Tier Credits as they will help you activate a Limited Tier for a player on your squad.