As Courtney said....With clarification, This game conforms to the "one Ball Condition" rule found in the USGA rules Appendix I; Part C; Section 1c. which states;
c. One Ball Condition
If it is desired to prohibit changing brands and models of golf balls during a stipulated round, the following condition is recommended:
“Limitation on Balls Used During Round: (Note to Rule 5-1)
(i) “One Ball” Condition
During a stipulated round, the balls a player plays must be of the same brand and model as detailed by a single entry on the current List of Conforming Golf Balls.
So as Courtney mentioned during your WGT round if you are on your last ball during your match the game will give you the option to purchase the same ball you are using for the completion of the round. If you opt not to purchase any then when the balls life has depleted or you lose the ball it will revert you to the starter balls to complete your round.
Hope that helps explain it.