"Plenum" refers to the air-handling spaces in buildings - typically the space above a dropped ceiling or below a raised floor used for air circulation. In these areas, regulations require cables with low-smoke, low-toxicity jacketing, to reduce toxic fumes if there's a fire. Frequently traveled areas may need more rugged or protected cabling but not specifically plenum-rated. Outdoor environments demand weatherproof or UV-resistant cables. Inside conduits often allow regular PVC-jacketed cable because the conduit provides fire containment.