3D canvas versus 3DX canvas

I can't figure out the relation between the 3D canvas and the 3DX canvas and how you're supposed to use them.

1. How do I import a STEP model, such as a heat sink, in the 3DX canvas? Only way I've found is to use the 3D canvas for the import and then the imported model seems to appear in the 3DX canvas as well.

2. A 3D-model of a symbol is sometimes only visible in the 3D canvas and not in the 3DX canvas. Why?

/F