The APA is the North American association that set's the "voluntary" standards for plywood over here.
The BS numbers are "British Standards" that are recognized by Lloyds. There are a couple of other European systems and a Russian system as well, but these others rely on the addition of the BS designation to indicate "suitable for marine use" - so you should see a Russian or European grade plus the BS number on an imported panel in the form of two different stamps if it's being sold as an "imported marine panel" (most panels from South America/Asia use the European rating system as well - so a combination of letter grades and BS numbers).
When looking for "domestic" marine plywood, you should generally be looking for APA stamps/ratings of A/A Mar (marine) grade or A/B Mar or B/B Mar (there is no valid marine grade domestic panel graded lower than a "B" - so if someone is selling an A/C marine panel, they are trying to pull one over on you) all of which would be close to the BS 1088 or (old) BS 6566 standards which often get tossed around when discussing wooden boats. But to save an arguement, the British Standards have more to do with the construction and durability requirements of the panel than simply the veneer quality on the faces - such that you will often see BB/BB "mahogany" panels with either BS1088 (meets all standards) or BS6566 rating (generally meets the standard but has some flaws within an allowable percentage). Over here if the panel fails to meet the marine guidelines in any respect, it's simply regraded as an exterior panel.
For a domestic panel to be rated as an A/A A/B or B/B, both face veneers must meet the "A" or "B" quality guidelines (or one A face and one B face for the A/B), inner plys can be nothing lower than B grade and all the plys must be either Douglas Fir or Western Larch that was grown in either the states of Washington, Oregon, California, Idaho, Montana or Wyoming or the provinces of Alberta or British Columbia.
Even if the panel otherwise meets the quality standards (and there's a pile more than I indicate above), if the wood did not come from one of the recognized states or provinces, then it can not be rated as a "marine" (mar) panel - it would simply be an A/A or A/B Ext (exterior) - although once rated as Ext the interior plys can also fall to C grade.
So while an Ext panel "could" be identical to a Mar panel in terms of construction, glue, process etc (although not likely - mills don't make money by making a product vastly superior to the standard), the marine panel will always be the "finest" quality (not that, that means a whole lot anymore, but it is the best domestic panel on the market).
Yes, Plywood is "real" wood
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it