SCSI tape confusion - what are the "modes"

[Tom Zerucha <tz@xxxxxxxxxx>]

I have a new SCSI tape drive, and when I am using devfs, I get four
device types, one each for modes 0-3 (and a "norewind" variand for
each).

What is going on?  I thought I needed mt to set things.  Also, I can't
seem to find anything linking the mode numbers to what a tape drive
actually does - I can imagine different compression levels and such,
but it isn't in the SCSI-howtos I could find on the web.

What are the a, l, and m variants of mt?