From the NEMA brief description document:
Type 12 - Enclosures constructed (without knockouts) for indoor use to provide a
degree of protection to personnel against access to hazardous parts; to provide a
degree of protection of the equipment inside the enclosure against ingress of solid
foreign objects (falling dirt and circulating dust, lint, fibers, and flyings); and to provide a
degree of protection with respect to harmful effects on the equipment due to the ingress
of water (dripping and light splashing).