The Free On-line Dictionary of Computing (30 December 2018):

SCSI ID

    The unique address of a SCSI device.  SCSI IDs
   range from 0 to 7 for 8-bit SCSI systems, 0 to 15 for 16-bit
   and 0 to 31 for 32-bit systems.  The SCSI adaptor is
   usually assigned ID 7.  A device's SCSI ID is often set by
   switches on the device.

   (1999-09-01)