Where the name of this software comes from
In bookbinding, a section, gathering, or signature is a group of sheets folded in half, to be worked into the binding as a unit.
A quire is the unit a book is actually made of: not the page, which is a face, and not the sheet, which is raw stock, but the folded gathering that gets sewn. Everything about imposition follows from that one fold. Kept here because the post on signatures says the same thing at length and this is the sentence to reach for.