Bolts are what holds it all together. There are always numerous bolts of various sizes used on mobility scooters and power chairs. Some are used as the axles on caster wheel assemblies, while others may hold the electric motor to the transaxle, or one of many other uses and functions.
A bolt will usually have an Allen or hex head, a partially threaded shaft, and is turned in by a wrench. Strictly speaking, a bolt should always be used in combination with a nut and washer.
A machine bolt is simply a bolt with a fully threaded shaft. Depending on its use, a machine bolt may or may not require a nut.