One is pressure treated (usually with CCA) and is for outdoor use such as deck joists, decking, Ground contact, planter beds.....CCA is slated to be phased out for less toxic(?)materials
The other is a normal 2x4 cut from a Spruce, Pine, Fir timber stand and is of "stud" grade. Used in standard 2x4 wall framing, and non decay resistant applications.
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