It doesn't
NEC 300.3(A) is basically saying all wiring methods are prohibited unless they are described in a chapter 3 article, so for example wiring with flexible cord is not an approved 'wiring method'.
So permissible wiring methods are in articles 300 - 399.
All chapter 3 wiring methods except Article 394 and 398 prohibit a single conductor to be run in a wall.
110.3(B)
Do you have a copy of the UL white book?
Its a free download.
