Difference between SCSI ID and logical unit number?

[Chuck Henkel]

A connection address for a SCSI device has the form AA-BB-CC-DD where
DD, according to info, is the SCSI ID and logical unit number [LUN],
in that order.

So what is the LUN? Is it always 0? The SCSI ID is set on the device
itself; is the LUN also? 

--
| Chuck Henkel                      | There are currently 111 operating      |
| Department of Nuclear Engineering | nuclear power plants in the US,        |
| N.C. State University             | generating nearly 100,000 Megawatts    |
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