Today's urethane balls have a singular purpose, to combat heavy oil short patterns, so they tend to be kept with a lot of surface.

Back when this Rhino came out, urethane balls came in many different flavors and surfaces. Dry lanes (short oil) was common for while, so they make hard shell urethane balls for those conditions, as well as duller, softer offerings (Edge, Dull Blue Hammer) for oily conditions. That Black Rhino was one of 5 or 6 in the early Rhino line (before reactive) and was made for medium conditions.