POSIX device number, PN. A PN is mandatory if the file type recorded in the "PX" File Mode field for a Directory Record indicates a character or block device (ISO_ROCK_ISCHR | ISO_ROCK_ISBLK). This entry is ignored for other (non-Direcotry) file types. No more than one "PN" is recorded in the System Use Area of a Directory Record.
See Rock Ridge Section 4.1.2