A hole in the ball that is not used for gripping purposes and is not considered a vent hole.
Often referred to as balance holes, or weight holes, Extra holes were originally intended to keep the ball within static weight limits set by the USBC (then known as the ABC). This is no longer the case, though a ball with a extra MUST be within static weight limits.
Extra holes are now an important part of the layout on a ball. According to Mr. Pinel, the extra holes account for up 84% of ball motion.
With the help of Steven Freshour, Mo Pinel created the Gradient Line Balance Hole System, which allows us to place an extra hole in specific places on the ball and accurately predict what the final ball motion will be. Using the Gradient Line, we decrease ball dynamics by 29% (P1), or increase ball dynamics by up to 55% (P4).
Extra holes have direct effect on the intermediate differential and total differential, which in turn affect the differential ratio of the final ball.
See: Gradient Line Balance Hole - by Mo Pinel