Sorry- that last response sounds kind of rude when I go back and read it. At my local plywood supplier, you can get certified underlayment, and non-certified. Flooring manufacturers often require certified underlayment for their warranty to apply. The certified has no knots or voids. The non-certified looks identical, but I guess it could have voids. They're only a couple bucks different. It used to always be luan, so it was for most intents and purposes, the same as non-BS1088 marine plywood. Lately, though, there's been quite a strong movement away from tropical hardwoods because a lot of it was illegally harvested. So a lot of the certified is now birch, and the luan is "labelled" (or advertised) as "certified plantation-grown, legally harvested tropical hardwood." At my supplier 1/4"x4'x8' non-certified Luan is $9, and the same dimension in certified Meranti with luan core is $11. They occasionally stock the Patriot Sure-Ply, and it's right in the same ballpark, but they just added a bunch of certification upgrades, so that probably means the price has gone up and the supplier figured that their customers won't pay for the price they're selling any more so they're not stocking it.

Sorry if the last one sounded snippy.

m