As mentioned, Annex B is not technically part of the Code, but is referenced in Informational Notes in several spots and is helpful when considering Code Sections such as 310.14(A)(3) Temperature Limitation of Conductors.
No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a with respect to type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded.
In my experience, if you are designing to Code minimums, you generally do not have to consider low voltage UG feeder derating (although an overzealous AHJ might invoke 310.14(A)(3) to force it. When designing for reliability, most Engineers will use Neher McGrath, in which Annex B Tables are prescriptive typical applications of those calculations.