Imposed formats
The imposed PDFs distributed on the library are meant to be printed and folded on the paper size specified. So the "a4 imposed" is meant to be printed recto-verso on A4 paper, folded and clipped, while the "letter imposed" is the same, but for letter paper, standard in the USA. There's little or no binding correction (whenever the binding eats space or not), because the clipping (with a long harm clipper) takes almost no space.
Example:
If the number of pages is greater than 80, folding becomes nearly impossible, so the books are split in "signatures". Example:
You just need to figure how many pages create a signature (the signature creation is automated). Usually is enough to see the first page, add 2 and divide by 4. You should get the number of sheets to fold together once they are printed.
Example:
Total pages: 280, i.e. 70 sheets. You simply can't fold 70 sheets. So the book is split into signatures. Look at the first page of the pdf. It's 54 and 0 (the first and the second pages aren't numbered). Take 54, add 2, (56), divide by 4 and get 14. Print it all recto and verso and fold them into 5 booklets of 14 sheets each. Then glue or bind them together and you have your book.
Plain formats: A4 and letter
Flat A4 (together with its US format, "letter") is a waste of paper if printed and is ill-suited for online reading: the pages are simply too big. So the idea is to use just half of the physical paper.
The A4 formats actually refers to A5, and should read "a format that you can use to print on a A4, side by side, as you please". From our speculations, it's probably a format suitable for Print on Demand services, because it has a wide binding correction (i.e., the space stolen by the binding near the spine has been taken in account). It's also suitable for online reading because the pages aren't too big and the hyperlinks are all active.
The same goes for the "letter" format: it's actually half letter, but suitable to be printed on a letter, 2 pages side by side, and with a wide binding correction.
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