Learn more about the  Vehicle Mechanical System
The term 'vehicle mechanical system' is pretty broad. Basically, the phrase covers nearly everything on your car that isn’t electronic or hydraulic. The engine, drivetrain, exhaust system, steering and suspension are primarily made up of mechanical components. Therefore, they would be considered mechanical systems.
But other parts of your car, such as the ignition system, are mostly made of electronic components. As such, they would be referred to as electrical systems.