NEMA Type 6P enclosures are constructed for either indoor or outdoor use to provide a degree of protection to personnel against access to hazardous parts and to give a degree of protection to the equipment inside the enclosure against ingress of solid foreign objects (falling dirt). Also, they protect the ingress of water (hose-directed water and/or during prolonged submersion at a limited depth). Lastly, a NEMA 6P rating provides additional protection against corrosion, which will be undamaged by the external ice formation on the enclosure.
In short, a NEMA 6P rating must keep not only people, dirt, dust, and debris from entering the enclosure but keep the internal components dry from water even if the 6P Enclosure is underwater for a prolonged period (24 hours).
Although UL has an official NEMA 6P testing procedure, we took the liberty of doing a simplified home version for the submersion part of the test. Click here to watch it.