You are confusing DWF and DXF (Drawing Web Format AND Data eXchange Format).
DWF is a read-only version of a DWG file (=AutoCAD DraWinG) - much like a PDF, that is a read-only version of a DOC or similar file. A DWF can be converted to a basic DWG with certain software - much like you can extract the text from a PDF if you have some tools. It can then be used in SUp with limited data.
DXF is an old type of a 'text version' of a DWG file (=AutoCAD DraWinG). You easily can open and read a DXF as text, and see what is meant to go where on which layer etc - it IS complex BUT it's still decipherable. It is simply a text version of the binary DWG file - so it's much bigger as the data is split out in easier to read coded packets. DXF is used for data transfer between programs that can't directly read DWG format. By now many programs (like SUp) can read native DWG - unless it's the very latest version where they play 'catchup', BUT many programs still allow DXF import - other programs (e.g. Bryce) only export in DXF so it's good to be able to get that data in even though it's not as 'compact' or perhaps as 'clean' as DWG...