Yes I should have clarified THHN/THWN. i've always seen it marked together. It would be at the installers discrection to determine damp or wet location and then provide the right materials for the job.
I must say that most deck wiring that I've done lately would be visible from below. A house on a hill with a ground floor entrance and a second floor deck, hence my willingness to use PVC. My thoughts of UF and standard steel staples, that might one day rust out and allow the cable to fall away from the structure was my reason to use PVC, two hole straps and stainless steel screws. Alot of my work comes from referrals and to have a potental customer see a professional job several years down the road, would encourage me to go the extra mile so to speak and bid the job as I have done.
In a ground level deck situation where traffic disturbing the wire may not be an issue, and economy is a concern the UF if attached with the two hole plastic straps and stainless screws should do fine.
Sometimes a code installation may be acheived through several avenues, mine may seem overkill to most but short of RMC I feel comfortable with it.
