Class C planets are small, barren worlds, utterly devoid of just
about everything: no atmosphere, no water, no geological activity,
and absolutely no life.
While the vast majority of geoinactive worlds are actually moons in
orbit of a larger planet, make mo mistake, they are planets in all
other regards. In fact, most have a semi-molten iron core, and
many are far larger than the bona fide terrestrial planets that
inhabit their star system.
Moon or not, some geoinactive worlds are born into the
classification, and spend their entire existence within the
boundaries of that definition. Others begin life as a highly volatile
Class A: Geothermal planet; once the volcanic activity eventually
ceases, those worlds cool off and become geoinactive.
Class C planets are often rich in minerals, and while they are not
suitable for life, they can be transformed into mining colonies via
the use of pressure domes. Some Class C worlds, such as Earth's
moon, are colonized in that manner just for the heck of it.