Purchase menus occasionally gate their best options behind play. A premium tier that guarantees the enhanced version of a feature may be greyed out until the player has triggered the standard version a certain number of times, or until an in-game achievement has been completed, turning the buy screen into a progression system of its own. Among unlock free spins slots with this design, the top purchase is a reward for engagement rather than a product available from the first spin. The rules describe the condition.