The Formation will be enabled at Lv 15.
You need to choose the Formation for the heroes you recruit via unlocking Starsoul in order to make them fight together with you.
Click the hero avatar and the Select Combat Slot interface will pop up. You can place the selected hero in the specified Combat Slot. There will be a “Battle” tab on the left of the hero that is already in the Formation.
Tips: After knowing the type of the hero, you can place the Defensive hero in the front row in order to protect you more effectively.
The Assist Slot will be enabled later. Once you light up the corresponding Starsoul, you can place the resting hero in the Assist Slot to offer Buff for all teammates.