Hi bartosh well a NURBS surface is made up of a "row and column" grid of control points so every surface inherently has 4 edges (some of which may be collapsed down to a point). But in NURBS modeling there is the additional concept of and "underlying surface" and "trim edges", which are boundary curves that live on a surface and can cut holes in it. The combination of an underlying surface along with trim curves makes what is called a "face". So you produce something with more than 4 edges by some process that makes a trimmed face. That can be a command like a boolean or also some commands build surfaces with trim curves from the start like Construct > Planar.

There's some more information on how NURBS modeling works with "underlying surfaces" in this FAQ entry here:
http://moi3d.com/wiki/FAQ#Q:_Why_does_show_points_work_for_some_objects_but_not_others.3F

- Michael