The special device is used for identifying your filesystem driver so that it can serve as a target for mounts and other operations. For example, if you have two CD-ROMs, they'll show up as /dev/cd0 and /dev/cd1. When you mount one of those drives, you'll need to specify which drive to mount from. The special device name is used to make that determination.
Together with the unnamed mount registration, the special device registration lets you get the mount message and correctly determine which device should be mounted, and where.
Once you've made that determination, you'd call resmgr_attach() to perform the actual mount point registration (see the next section below).
The special device is registered with a valid name given to path (for example, /dev/ramdisk), a flag of _FTYPE_ANY given to file_type, and a 0 passed to flags.