Cam Duct and Est Mep can run by themselves.  Fabrication Cad MEP needs AutoCad and/or AutoCad Verticals to run.

Fabrication Cad MEP is also a plug in to Revit but some of the database management must be made from within AutoCad.

You typically should not purchase Fabrication by itself.  At a minimum you would  need to purchase Fabrication and AutoCad together.

Somebody else can chime in and correct me if I am wrong.