The Polyline command is used to draw complex lines - these are single entities consisting of a number of line segments. Various editing functions can be applied to polylines making them a valuable design tool. they are commonly used to delineate areas of ground cover planting and areas requiring hatching such as paths and decking, but are also useful when creating your own symbols.
The figure below shows a polyline delineating and area of what will become decking. Each vertex (change in direction) of a polyline is numbered and can be easily edited.
An area defined in this way is easy to hatch.