Definitions can vary somewhat, but I consider low RG balls to be in the range of 2.45 to 2.53, medium from 2.53 to 2.59, and high RG from 2.60 to 2.80. As a general rule, a lower RG ball will be a bit easier to rev up and will tend to hook sooner than a higher RG ball, given identical coverstock and layout. A higher RG ball will be a bit more difficult to 'rev-up' and will tend to go longer down the lane before wanting to hook. However, RG is not the only determinant of length. Coverstock and layout will also effect length. Duller balls want to start hooking earlier, and track flare will make a ball roll/hook earlier. All of these 'ball' factors affect length. There are 'human' factors that affect length, but you didn't ask about that.
