Say you have a long run (300Ft) part of that run goes over a Hot area blast furance ect.
The part over the blast furance Must be taken into account for the total run (Derated because of Temperature). See table 310.16 Correction factors at the bottom)
See Exceptions for areas Beyond the point of transition .
10Ft or 10% or less of circuit total lenght the highter Amerage rateing can be used.
If run is 10ft or less or Hotter area less then 10%
2005 Handbook has example & clearly explains it.
As far as 90 degree rateing on conductors Except for HV cable that Amperage is to be used only for derateing of conductors.(see 110.14(C).
