The difference of the "THHN or THWN" type conductor is the temperature rating of the conductor for ampacity rating.
If you use the "THHN or THWN" conductor (dual rated conductor) for underground, you can only use the THWN using the 75*C column of table 310.16, even for ampacity adjustment.
Example: 2 AWG THHN is rated for 130 amps at 90*C, 2AWG THWN is rated for 115 amps at 75*C.
