Joining vs grouping polysurface

Hello, I have a question.

In Rhino 5. I have some triangular surface objects that I want handle as one object.

I can join this objects or I can use group but behaves differently.

My question is when I join them why do I lost the individual materials and get one single materials on all object ?

Another thing, when I join the surfaces the object type turns from surface to polysurface type.

Join is designed to create a single object from multiple objects, such as creating a single polysurface from multiple separate surfaces and/or polysurfaces. A single object can only have a single material associated with the object.

Group tells Rhino to treat a set of objects as a single entity without creating a new object.

Yes, This is what I wrote. The question is, why join modify the object type from surface to polysurface.

That is because you end up with a surface built of multiple surfaces, a polysurface. You can explode a polysurface to get the constituents.

Thank you. It makes sense now. Poly = many (as polymorphism ).

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