The law states any lumber touching concrete or exposed to the elements must be weather resistant. This means the following species of lumber can be used: Cedar (from the west to east coast), Redwood (California), Ironwood (the Amazon jungle) treated (any species of wood usually pine is treated).
Buying treated lumber at good (High end home builders shop) lumber yard will deliver a quality product the problem comes in when you have cheap mega stores also selling lumber these places make their own treated lumber.
They buy a train carload of lumber and push it in to large basined of Chromated copper arsenate (CCA) is a chemical wood preservative.
Let it set for a number of hours then pull it back out. They pack it up and ship to off to the store.
The good lumber yard have their lumber pressure treated with CCA pushing into all the cells of the wood 40% of treatment.
An easy way to tell is if you cut the board and look at the end grain is the treatment through out the end cut.
60% of treatment of CCA is used for wood foundations buried post footings, etc.