Technically you can probably call it a rounded isosceles trapezoid but it will be more useful as a base for path of bicycle chains or pulley belts. When both radii are the same and spline set to renderable, it could also be used as a simple chain link.
Note: When the parameters would result in impossible shape (such as when the sum of the smaller radius and width will be smaller than the bigger radius etc.), the spline will not be drawn.
Installation: Copy the .ms file to your scripts\startup folder or run as a script if you want to try it out first. The object primitive will be available under Shapes -> Examples dropdown menu in the Create panel.