USBC Rules.
"Surface:
The surface of the ball shall be free of all depressions or grooves of specific pattern, except for holes or indentations used for gripping the ball, identification lettering and numbers, and incidental chipping or marking caused by wear.
Engraved pictures are not considered grooves of specific pattern.
No foreign material may be placed on the outer surface of the ball.
If the surface friction of the ball is altered by sanding or polishing, the entire surface of the ball must be sanded or polished.
Plugs and grips may not extend beyond the surface of the ball."
Equipment Spec Manual-Page 13 (Jan 05 Revision)
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
PBA
It is very popular on the PBA to hit the back flares to promote hook in the back-end, especially on carrydown.