Traditionally sewing machines have been made by adding one component at a time to the frame. The motor is mounted and linked to the upper shaft with a drive belt. The upper shaft drives the cam stack and needle bar. A lever, vertical shaft, or a belt link to the lower shaft which drives the feed and hook systems. If a gear, lever, or other part wears out, breaks, or otherwise requires replacement, it is as simple as replacing the one part or maybe two. Unfortunately, this usually required moving a bunch of other items out of the way. Then once the parts are replaced, you must reset everything - needle bar, feed, hook, etc.
With the advent of modern computerized sewing machines and manufacturing advances, it has become common to make the machines in modules. If a module breaks, just replace it. Often this can be done without having to reset every aspect of the machine because the module is set correctly before installed. You should double check, but if installed properly, it should work.
Apart from the electronic modules, the needle bar and presser foot assemblies; hook and feed assemblies; and drive assemblies are often manufactured in easily managed modules. This makes repairs much faster and easier. For example, a lower drive module may include the hook assembly, feed assembly, and drive shaft.