Extend adds a linear extension to the selected surface similar to extruding the boundary curve of the surface however far you set. The direction of the extend is the tangent direction at the boundary. Remember that the surface is defined with rows and columns as you select which boundary to extend.
Follow these steps to extend a surface:
Extend requires an existing surface to work with.
Use the Surface wizard or select Extend surface from the surface toolbar flyout.
Set whether you want to modify the existing surface or create a new surface from the process.
If you are creating a new surface, name the surface for future reference.
Click Pick and select the source surface, or select the surface from the dropdown list box.
Select the edge to extend either with the radio buttons, or click Pick and select the edge with the mouse.
Set the distance for how far to linearly extend the surface.
Click Finish or OK to create the new surface.
More about extending surfaces.
An example of an extended surface.