With this switch you can toggle between different ways of treating the device. You can choose between TAPE, FLOPPY and BLOCKDEV.
I recommend using this format for any removable media, like floppies or removable hard-drives [+] or cartridges. It does not work with tapes, it is intended for block devices that can be mounted. The BLOCKDEV support was new in version 1.2.0 and is therefore less tested than TAPE of FLOPPY. If you should expect any trouble using it, please contact the author. If it works for you, I'd like to know as well.
However, using this format is not recommended, better use the BLOCKDEV--format which is more reliable.