A scroll bar is used when the content is beyond the viewing area. The content may be horizontally or vertically spread beyond the viewing area. In such situations a scroll bar is provided to allow the users to scroll either vertically or horizontally to view the remaining area.
To notify the screen reader users about the existence of a scrollbar on the page the scrollbar role is used. Since the scrollbar will have a viewable area on both sides i.e. left and right for horizontal scroll, top and bottom for a vertical scroll, minimum and maximum scrollable area should be intimated to the user with aria-valumin and aria-valuemax notifying the current value with aria-valuenow. Aria-orientation property can be used to notify the user about the direction of the scroll.