Made to order means the window starts with your opening, not a catalogue size. It might be a casement for a narrow bathroom wall, a run of sashes for a front room, a wide sliding unit onto a deck or a fixed pane set high in a stairwell. Each one is built around the type, frame and glass it needs.
With no standard range to squeeze into, the conversation starts with the opening and what it has to do. It might need to open, let air in, keep noise out, hold heat or simply look right on the house.