The PT means it was pressure treated, that's the only way they treat southern yellow pine, you can tell by the color of the wood also. Usually there will be a sticker on the ends of the boards indicating the amount of chems in the wood with .15 for above ground contact & .40 for ground contact. All your post should be rated at .40
In some of the western states they treat fir & hemlock with incisions in the wood & soak it as pressure treating doesn't work on that species of wood.
The number 2 is the grade of the yellow piine board and has nothing to do with the treatment. Prime means less wane on the edges than regular #2 grade allows. I'd bet a cnote that board came from Lowes.
The number 283 indicates which mill the board was milled at. Most mills do not treat lumber, that's done at another location.