Extendable & Double Poles
Extendable poles are designed to cover a width range without cutting, which suits standard windows where you would rather not commit to a fixed length. Double poles carry two curtain layers on one fixing.
Layering usually means a main curtain with something lighter behind it, so the projection of the brackets matters as much as the pole. The front curtain needs room to hang clear of the back one.
Check the stated width range and the bracket spacing for the specific system before ordering, rather than working from the pole length alone.