What are the dash marks in the outdoor lumber in phoenix for?

[Colleen F.]

Ok so in RI we use pressure treated wood a lot outdoors for decking and such,  but here in AZ desert area you seem to use dimensional lumber for everything. I know you don't need pressure treated in the dry areas.  However, the wood for decking and stairs etc has these little dash marks in it. They obviously put them there for something but I have been looking and can't find an explanation for what this is called and why they do it.  I think it may have something to do with drying but just throwing it out there for the construction experts. Anybody have an explanation?