The purpose of the unnamed mount registration is to serve as a place to “listen” for mount messages. These messages are generated from the command-line mount command (or the C function mount(), which is what the command-line version uses).
To register for the unnamed mount point, you supply a NULL for the path (because it's unnamed), a _FTYPE_MOUNT for the file_type, and the flags _RESMGR_FLAG_DIR | _RESMGR_FLAG_FTYPEONLY for the flags field.