Extrude surface command question

I’m trying to extrude the surface on the two surfaces in the attached file.

When I start the ExtrudeSrf command, I am only able to select small sections of the surfaces, rather than the complete surface.

Can anybody explain why this happens?

extrude surface test.3dm (3.4 MB)

That’s because they are not surfaces, they are open polysurfaces. To use _extrudesrf you have to use _explode first and then use _mergesrf to get 1 piece of it.

Thanks very much for the answer!