Not that it matters much, but I think it's due to the way pages are printed and bundled together into sections (graphs?). "Signatures" is the correct term for the bundle of pages and "Imposition" is the way the pages are laid out on the sheet. I spent 10 years as a printer/typesetter. In imposition, if you wanted a page left blank, you just left it blank. I think labeling "page left blank" deals more with the expectations of the user or reader of the book than of the printing process involved. Paxton Astoria, OR