The key point about the word "deal" is that it originates from "plank"
or "floor".  The Oxford English Dictionary (OED) traces its origins
back to Low German "dele", and points outs similar meanings in modern
Dutch.  It's intriguing to speculate whether there's a common origin
with the card-dealing sense of "deal" as dividing, although OED has
this down as a different root in English.

The OED is extremely precise as to the limits of "deal", being a slice
of timber at least 6 feet long, 9 inches wide and not more than 3 inches
thick.  If it's narrower than 7 inches then it's a batten!

The wood used for these was the original question, of course.  OED has
"White" deal from Norway Spruce (Abies excela), "Red" deal from Scots
Pine (Pinus sylvestris) and "Yellow" deal from Yellow Pine (Pinus
mities).  I would expect that many others were used as well.  From what
I've read of the modern domestic trade, such clear distinctions of
softwood by species are mostly a thing of that past and you have to
take whatever you are given!

Julian Dyer