In the following, we limit our attention to phenomena that take place in inertial frames, and in which rigid objects, forces acting upon them, as well as rays of light are involved. A clear distinction is made between rigid objects and light: the latter is considered more like a signal, not as an "ordinary" physical object. Elastic solid objects (threads, springs, etc.) are also allowed, but used only as a means to exert forces on rigid objects in a measurable way.
All in all, the intended scope is the most general as far as the motion of rigid objects in inertial frames is concerned: there are no restrictions expected that would limit the applicability of the results obtained later.