I could not find any elaboration related to 3D Shape concept anywhere in the 3DEXPERIENCE Help.

I do not understand the purpose of automatically having an object- 3D Shape in the model tree whenever an empty part is created. We can not have more than one 3D Shape in a 3D Part; we can not use the sole 3D Shape anywhere (can not move it into any 3D Part/Product), so why it's just there in the model tree? What's its purpose/specialty?