Ron,
Although this hasn’t come up in any conversations I’ve had, I would guess it is like a normal runner area rug except that it is on a stairway instead of laying flat. Prior to my carpet cleaning days, I have seen carpet on stairs that did not cover the entire width of each step. The steps were usually hardwood outside the rug’s edge over to the wall. Of course a stair runner would have to be secured if on a stairs rather than just laying loose.
In case you’ve not heard of a normal runner area rug, it is just a variation of an area rug that is long and narrow usually put along traffic lanes in a house.
Mike