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The first word in the name of "AutoCAD Plant3D" it's AutoCAD. That's mean that when you go to install Plant3D 2013, the program it will install the AutoCAD2013, automatically. It's imposible to install only Plant3D without AutoCAD.
If you have AutoCAD 2012, and go to install Plant3D 2013, your PC will have two versions of AutoCAD simultaneosly 2012 and 2013.