Ruben,
Here is an explanation from Robert Lansdale of Okino Computer Graphics:
"What you are experiencing is all part and parcel of the MCAD manufacturer's pulling the wool over your eyes, as the terms I use. PTC, Solidworks and others have a well defined "hack" (which I too have on my IGES exporter) which fools people into thinking that mesh data can be "imported". That's what causes forums around the world to explode with incorrect perceptions towards this process. These are called "proxy graphics" in which their hack allows a mesh model to be viewed but not partake in any modelling operations. ProE/Creo is purely a BREP solids modeller and hence has no understanding nor notion about polygonal data. To edit polygonal data you need to use a very simple modeller such as Rhino, 3ds Max, Maya, SketchUp, etc. and not a high-end and common MCAD modeller."
