According to Retiredff :

Chris Lewis wrote:

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.

Just curious- where the heck are you? Like I said, I've never seen any like
that. Then again, I've never looked outside of the mid-Atlantic region.

Ontario, Canada. Some lumber is relatively local, rest is from the west
coast.

The slashing is a "well known". It's mentioned in a lot of US books.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.