With the rotary ball spline, the spline shaft has three crests, and along both sides of each crest, two rows of balls (six rows in total) are arranged to hold the crest so that a reasonable preload is applied.
These models are unit types based on the Model LBR, but have gear teeth on the flange circumference and radial and thrust bearings on the spline nut, all compactly integrated.
The rows of balls are held in a special resin cage so that they stay aligned and recirculate. With this design, balls will not fall even if the spline shaft is removed.