This is also a license that you need to purchase and add in the Registration Section. For this you also need the VTL frame license if not allready in use.
the 50TB addon can be used to backup direct to disk or stage to disk then off to Tape.
If you don't have a VTL Framework Registration you can just allocate disk from your SAN say and create AFTD's and add the AFTD to networker as drives.
you could also create mount points and add these as AFTD's in Networker.
This is really easy to do and the application Help and the PDF doco have plenty of info in then to guide you. Personally i am a fan of the AFTD advances file type devices and use them quite a bit. combine them with staging and set your water marks and then you'll have them cleaning them selves out daily/weekly ect...