Well… If you really would like to dive down into ancient descriptions and origins, I have a few links I could send you 
I am brought up with Swedish terminology, so that is why I immediately know what a, say, “40 grams papper” means, feels like & is used for. Having worked with ad agencies and printers of all sizes for decades, one has to learn the lingo. And I guess it is exactly the same for an Imperial AD, whose life rotates around Text, Book, Elephant, Fool’s Cap (a.k.a. foolscap), &c.
Now, in order to try to make life a trifle easier: when picking papers for your cal, would you prefer to think in Metric terminology or in the Imperial one?
If you think that this is a quagmire, think of the poor old Italian alchemist, who found a recipe for Real Gold™ in an old manuscript: you take a dram of this and a dram of that and admix a pound of something. Problem was that he had to know in which Italian city the manuscript was penned, since a pound in those days meant different weights in different districts.
Oh, joy!
Have fun!
Typographically Yours,
Claes in Lund, Sweden