The logical number of the file open for output by the current graphics device. This field only has meaning if the file is accessible to the user from PV-WAVE, and is 0 if no file is open. For example, the PostScript driver maintains this field with the unit number of the file open for PostScript output. In the case of Tektronix output to a file, !D.Unit may be set to either + or – the logical unit number.