RG numbers tell you how cover or center heavy a ball and its core are (high RG = late reaction, better for dry; low RG = early roll, better for more oil as a rule of thumb). RG differential tells you how much the ball track will flare on the lane, and/or how strongly it will tilt. It gives you also information about the hook potential of a ball. The higher the differential, the higher the hook potential. High RG balls are better suited for more/longer oil, since a high RG is better suited to keep the ball's track on a dry ball surface area when it goes down the lane, increasing traction.
These are only rough guidelines. The ball surface and drilling will also have great effect on the ball recation, but RG and RG diff. give you a hint for which general lane condition the ball was designed in the first place.
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DizzyFugu --- Reporting from Germany
"All that we see or seem, is but a dream within a dream..." - Edgar Allen Poe
