Exterior grade lumber

[Mike Lamb]

This lumber is being used for an outside deck. It looks like standard construction grade and nothing that should be used outside. Do these stamps determine location use?  I did a quick walk-by and will look at the house tomorrow.  

Typical pressure-treated lumber is tagged at the end of each piece with treatment info.

[John Kogel]

I think you are right, no mention of pressure treatment in those stamps.

HT stands for heat treated.

[Trent Tarter]

As far as I know there's nothing stating that it cant' be used. It just needs to be well stained and sealed. I see quite a few decks built with untreated wood, personally I would use treated wood for the joists and beams.  

[Jim Katen]

It's required by the IRC in section 317.1