This contains additional gears so you will always have the right combination available to ensure you can achieve the desired balance of torque, speed, and direction. All the parts included in this kit were also included in the Starter Kit, with the exception of the 84-tooth gear, which is just like the other gears, except with more teeth, which gives you access to even higher (or lower) gear ratios.  The 60- and 84-tooth gears have extra mounting holes. These holes can be used for attaching metal bars or parts to these gears when using them as part of a turntable system or other similar design.