A Macintosh SCSI device uses 3 Bits to address ID 0...7, a total of eight IDs to choose from. That is the reason, why you have three jumpers to set the ID. ID 7 is used by the host controller, leaving seven IDs free for other devices. You may attach up to seven devices with different IDs from ID 0...6 to one host controller. Internal and external devices sharing the same host controller also need to use different IDs 0...6, each. You may attach more than seven SCSI devices to one computer, if you have several SCSI host adaptors in the same computer.