This wiring method is the nonmetallic extension. While 380, which preceded this, doesn't have a section 2 (a rare thing), Article 382 does. There, we really find two definitions. First is for the nonmetallic extension itself. It's a listed assembly of two insulated conductors within a nonmettallic jacket or extruded thermoplastic covering. This includes surface extensions for mounting directly onto the surface of walls or ceilings [382.2].
If it's a concealable extension, the number of conductors may be two, three, or four. That's one difference. Another is it can be in a sealed nonmetallic covering (or in (in one of the other two jacket types from the previous definition). And you can conceal it with paint, plaster, wall paper, etc.
You can use it from an existing outlet on a 15A or 20A branch circuit, and only if it's exposed (or concealed as permitted in 382.15), in a dry location, and in a residential or office building no more than three floors above grade [382.10].
- There are four. You can't use it unfinished basements, attics, or roof spaces (that's one); where the voltage between conductors exceeds 150V (or 300V for arial cable); where subject to corrosive vapors; or where run through a floor or partition or outside the room in which it originates [382.12].
- Secure them in place using approve means at intervals no more than 12 inchines. Firmly anchor them to the wall using an adhesive or mechanical anchoring system identified for the use [382.30].
- All fittings, connectors, and devices shall be of a type identified for the use [382.42].