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what is the best enviroment in catia,for designing a car body?
ICEM Shape Design
Generative Shape Design
Starting in gsd would be a good idea. I couldn't imagine starting right into icem.
I don't know about ICEM Shape Design, but mostly I use freestyle and GSD to some extent.
If you just want to start a first styling process, Catia Shape sculpture ist a nice tool.
For serious and parametrc car body design, GSD ist the first and best choice.