“””””” I did some tests but the 3dm files saved with Moi3D surfaces are not imported while those saved with RhinoV5 are imported correctly,and wonder why?”””””

Rhino uses a concept of saving the “display render mesh”

They arent using NURBS. It is basically a polygon model

MoI does not save this mesh data in a 3dm.

You should be able to get the same results if you do a “save small” in rhino. This drops out the render mesh to save on file sizes.

I thought and assumed that the add-on also makes on fly the tessellation.

Some more details on what Burr mentions here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5875.3

Often times 3DM import plugins like this kind of take a shortcut and don't do any tessellation of their own. Instead they rely on a cached tessellation being present in the file. MoI doesn't write that because it significantly bloats up file size.

- Michael