I'm using bpftp server 2.15 tried Serv-U then switched. Usually it means what drive you give access to. When you set up a user account you choose what drive and directorys you give access too. ie if your main drive is C: and you want to give access to drive F: just add that path to the users access rights
F:\apps\ RW-AMLS Read,Write,-Append, Make, List, +subdirs. if you don't give access to sub directorys then F:\apps will be the only dir. they can access. do the same with other drives D, E, G, etc. click scan drives at start up to scan all available drives. Hope that was what you were looking for.