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.
Per PCBelarge' comments: don't forget that the "weakest link" in the circuit governs. The terminals at equipment usually are rated at 60oC or 75oC, so that is the limitation on ampacity. i.e. Column to use in Tbl 310-16ff. (310.15(B)FPN1
The "plumbing pipe" thing happens to me all the time. Customers call me after they have poured the patio and tell me they already installed the pipe for the hot tub to make things "easy" for me. It's usually 1/2 inch plumbing pipe and fittings no where close to where I need them. Thanks for the help!