The balls bought at the shop or claimed as a reward could be checked on 'BALL' at 'CARD' menu.
By marking the balls on 'BALL' menu, they are equipped in the in-game ball slots.
When selecting the ball inside the slot, the pitcher will start to throw the selected ball for the next pitch;
You can also change it to another ball during the match.

※ After the v1.12.1 update, Balls are unselected and not saved under in-game ball slots. You will be able to use it after marking it in 'BALL' menu