The wiring method in question is concealed knob and tube. Back when homes had a 30A service and the loads were just lights and a few receptacles (to which you'd connect really small loads like a radio), this was OK. Not much current and not many circuits. With this method, you run bare conductors from knob to knob (twisting the conductor around a knob to secure it) [394.2]. When we say "bare conductors," that needs to be explained in this context. The conductors are run in a raceway, which in this case is a flexible nonmetallic tubing. The conductors aren't themselves insulated; the raceway provides the insulation.
There are only two permitted uses [394.10]. You can use it for extentions of existing installations, and this the most common usage. For example, an old house has a knob and tube system and you want to add a receptacle on an existing circuit. When used here, this wiring method can be installed in the hollow spaces of walls and ceilings, or in unfinished attics and roof spaces. Keep this use in mind when we look at the prohibited uses. You can also use this system by special permission, though it is really hard to make the case that this method should be used when safer methods exist.
Article 394 provides five prohibited uses. As you've no doubt guessed, hazardous locations is one of the five. You also can't use it in commerical garages, theaters (and similar locations) or motion picture studios.
And there's one more place you can't use it. This gets us back to a subject raised in the introduction and in the preceding answer. If those spaces where it can be used are insulated by loose, rooled, or foamed-in-place insulating material that may envelop the conductors, this method cannot be used. This has implications for home rehabbers who seek to bring an old house up to modern energy standards. You can't just insulate the house if it has knob and tube wiring. You must replace that wiring, and then insulate the house. Yes, electricians normally don't get involved in insulating homes but you may be consulted on this question and now you know the answer.
- You can use tie wires to secure the conductors [394.30(B)]. Article 394 does not specify the hardware you can use, but it does specify support requirements [394.30(A)]. In the old days, the supports were often driven nails that were bent to form a cradle or saddle. Common sense tells us we can use a modern insulated (or nonmetallic) cable saddle of suitable size. Don't use nails and don't use straight staples. If your distributor can obtain listed support devices for the system, use those.
- This wiring method doesn't use couplings and connectors. You can't use in-line or strain splices. Oddly enough, the NEC requires you to solder all splices unless you use approved splicing devices [394.56]. Since solder can easily melt, we normally do not use it for 120V circuits. So try to avoid it with this wiring method; take the time to obtain approved splicing devices and (if they are compression devices) the tools to properly apply those devices.