Also another thing that helps putting each part into its own file is that you can then position each one at the origin, without having some big lump of objects all stacked up on top of each other which is what you'd get if you tried to have a whole lot of parts (to use with ImportPart) all contained within just 1 file.

That's because the origin of the construction plane (which by default is the world origin 0,0,0 but you can set it to some other place by using View > CPlane) is used as the base point for where to place the object in ImportPart.

- Michael