Used to extend an element (line, line string, arc, ellipse, complex chain or Bspline curve) by a certain scale.
You can access this tool from the following:
 Ribbon:
 Toolbox:
Modify Curves
Setting  Description 

Type 
Defines the type of the extension.
 Linear — The extension is a straight line.
 Arc — The extension is an arc.

Method 
Defines the method used to create the extension.
 By Point — The extension is defined graphically with a data point.
 By Distance — (Type set to Linear only) Lets you set a distance value for the extension.
 To Element — (Type set to Linear only) Lets you extend the curve to another element.

Distance 
(Method set to By Distance only) Lets you input a value for the distance to extend the curve. 