The 3D modelling softwares have different engines to compute the objects, they have their own file formats. The conversion between these applications goes usually in two ways:
Using DWG/DXF format: the content of the file depends on the conversion system of the original modelling software. This decides for example, that the 3D surface in the original file appears in the dwg as block, polyline, 3d primitive, region etc. In case of advanced softwares, you have options to set this.
Using 3DS format: this is an other usually used format to transfer 3D data. ArchiCAD has a 3DS format supporting add-on...
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