The ball tracks are the longitudinal grooves that run along the inner race (spline) and press against the balls in the outer race. Most sizes will have more ball tracks in the outer race than the inner allowing the user to install a new outer race on the inner spline and extend the life of the inner race.  Additionally, outer races are ordered according to how many tracks are loaded with balls.
Below is a list of the number of ball tracks according to the nominal diameter of the inner race:
Ball Circle Dia.Number of ball tracks .375''3 .625''6 1.0''12 1.5''12 2''12 2.5''12 4.0''18 5.0''16 6.0''24