Don’t be surprised when you see different treatment levels on pressure treated wood that are supposed to resist rot equally well. The numbers depend on the chemical preservative used in the treatment. Every treated board carries a label. Check it to find the type of preservative used. The preservative retention for both CCA- and ACQ-treated lumber is .25 for above-ground applications and .40 for ground contact. CCA use has been reduced, so you’re more likely to find ACQ when buying treated lumber.
However, the preservation retention is different for another common treatment, called CA-B. The CA-B treatment level of .10 corresponds to an ACQ of .25 and a CA-B .21 to an ACQ of .40.
When a project calls for a certain preservation rate, it’s usually for ACQ unless otherwise specified. Lumberyards don’t always carry each type of treated wood at each level of retention. The cost for a .25 ACQ-treated deck board is about the same as for a .40, so some lumberyards and home centers don’t want the hassle of carrying both. Instead, they often carry only ACQ .40.