This deck was built in the mid-Atlantic area in 2005 using ACQ-lumber treated for above-ground use. The 5/4x6 cap rail shows clear signs of premature rot.
The same 5/4x6 ACQ stock that was used for the cap rail in the previous photo was used for the decking, which also needed replacement.
Each stick of treated lumber has a label that identifies the chemical formulation that it has been treated with. Retention levels for different formulas and different uses are established by the AWPA or by code evaluation reports, such as ICC-ES reports.
Check your lumber’s end tag or ink mark to avoid inadvertently cutting stair stringers from PT stock that is only rated for above-ground use, and treat all cuts with copper naphthenate.
Ground-contact lumber should always be used for deck joists, beams, and other framing that is in close proximity to the ground.