A model file is one piece surprisingly rarely. This truck is five: a wheel on each side, and a body that is itself paint, glass and window trim. All of them are drawn, and one placement moves the lot.
model.parts fills in when the file arrives, and its parts are ordinary models, each with a material of its own. The name of each surface is on the loaded model (truck.parts), in the same order, and it is often the only thing telling two apart: here it makes the glass see-through without touching the paint.