The Bushings & Bearings enable your gears to turn freely and smoothly. Bushings and Bearings come in different sizes. Different gearboxes have different sizes, between 6mm and 9mm. This is the innerdiameter of the holes in the gearbox (Ergo, this is the outer diameter of the bushing/bearing, not the flange!). Larger diameters can handle more force.
Bushings: Bushings are solid metal, and are better at handling high torque, but turn less smooth than bearings.
Bearings: (Ball)Bearings turn much smoother, and are therefore better suited for high speed. They are however more sensitive to too high torque.