For many years I associated codex with ancient ~ written ~ archaeological finds. I was not wrong over that, but I had completely missed the point that codices was a descriptor of the binding technique.
A folio is a piece of paper ~ or similar material ~ folded into half. When a folio is sewn to other folios ~ either tucked inside ~ or adjacent ~ or both ~ then that is codex binding ~ or codex sewing. Historians may wish to reserve the name for handwritten early books ~ as in the paragraph above. It will be obvious to readers by now that I do not subscribe to that narrow definition.
Without the fold in the folio to provide the hinge for the pages ~ and to provide a groove for the thread to link the pages together the folios have have to be joined somehow ~ and in order ~ to make a useful book. Other alternatives are stab sewing [117565] or edge gluing [115742]. Other binding names ~ such as Coptic or Longstitch ~ are also in the codex binding family.