Linear curves consisting of straight-line segments are called Polylines or Polygons. From this description, you may think that you will want to use this type of input whenever you need flat areas in your model. This is not the case! There are better ways of making part of a curve straight. In fact you will hardly ever use Linear input – it is really only available for internal use within the software.
To interactively create a Polygon curve, set the input mode to Linear, and use the command CURVEÞ Design:Input to enter the polygon.