how are meshparts generated?

Hi, I have a CAD model which I constructed in Siemens NX9. It shows an UR5 robot placed on a assembly table(its precise position on the table is relevant).

I imported the whole model into Unity as .dxf and it looks pretty good. The model seems to be split into several meshparts(Robot, Table and other Stuff) but as I am planning to move the robot I would like to split every joint into a seperate meshpart.

Therefore my question: How does Unity decide to split the model into parts? Does it get its informations via the .dxf-File and I have to determine the parts already in NX?

Thanks for your help, Didi