I think in means "Random Length".
If used outdoors it should be be sealed with a good water proofing.
Many HDs due carry poplar, for I purchase poplar often. The 12" wide is not often in stores because the pieces easily cup and twist, which most people do not want to purchase. The pieces will sometimes cause problem in your project. Depending on the project, I will glue up narrower pieces to create the wide ones using biscuits for alignment. Look through the pieces to obtain a good match along the edge.
I'm sure that it could be special ordered in that length but the cost would be quite steep I'm afraid!
Yes it is! If they don't have a 10 footer they will cut a longer one down for you.