According to Retiredff :

AutoTracer wrote:

Thing I don't like about pressure treated (especially 2x4 and 4x4) is
the little grooves cut in the surface to identify it as PT. this
kind of ruins it for exposed work since you shouldn't sand this wood
extensively.

I have never seen any pressure treated lumber with grooves or any other
identifying marks cut into the wood.

Are you sure you aren't talking about the shallow scalloping that occurs,
from the planer blades, when rough lumber is sent through a planer too fast?

Depending on the manufacturer, depending on the size of the lumber, the
mill will "slash" the face of the lumber so the PT will soak in better.
It's not "identifying it as PT", but an aid to getting better soak of
the preservative in shorter time. Manufacturing codes require that
the preservative soaks in a given amount, and if the wood type/thickness
doesn't allow it in a reasonable time, they slash the lumber to improve
penetration.

Here, 6x6's and larger are always slashed. 4x4s sometimes. I don't
remember ever seeing slashed 2x lumber - at least not PT suitable
for decking or hand rails.

["High grade" PT rated for direct immersion might always be slashed even
in 2x, but that tends to be the underside of docks, so who cares? ;-)]

Here, the slashes look like a series of 3/4-1" long, very narrow,
slice marks aligned with the length of the board. Sort of like
someone did a lot of stabbing at it with a very thin jack knife in
a surprisingly regular pattern ;-)

Not at _all_ like planer scallop.

We use opaque stains for uprights, and transparent for horizontals.
Only large posts (uprights) are slashed (if any). Given the relative
thickness of opaque stains, and the fact that the posts seldom get
much human contact, the slashes don't present a problem (either to
the touch or in appearance). Indeed, the opaque stain tends to
make the slashes disappear.

The decking, which is 2x material, isn't slashed, so it isn't an issue.

If you're stuck with slashed 2x PT lumber, look elsewhere, someone
will have the unslashed type.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.