Returns the value of the closed flag. 

If the closed flag is set to true, the created surface starts and ends at the first cross section (i.e. the surface is closed). If all of the cross sections are closed, the resulting surface takes a torus-like shape. If the closed flag is set to false, the surface is opened or capped with a planar surface at the first and last cross sections. If the flag is set, then there must be at least three cross sections. 

The default value is false.