Revit System Families differ from Revit Loadable Families in that they cannot be saved to an .rfa file. System Families are stored in the Revit model (.rvt)
Examples are Walls, Floors, Ceilings, Roofs, Stairs, Ramps, and Railings.
dRofus will save these as .rvt files instead of .rfa so that the materials can be carried and integral sweeps are also loaded.
Upload and download workflows are the same as Loadable Families. The only difference is the file format is .rvt.