Logical Units vs Positions

Written on every Microtech backup tape we have is a nomenclature of Logical Units and Positions, such as "Logical Unit 0, position 1", or abbreviated:  "LU 0, pos 1;"... and so on, or some mixture thereof.

Click to see pictures of all tapes on Google Drive

I've been told by those more familiar with this vintage technology that I should think of Logical Units as hard drive partitions.  But then what do the positions mean?  

Google wasn't any help to me on this.  Searching the phrase that now is the title of this post revealed nothing that really matched.  So, I was left to guess.

And, well, maybe I was looking at it the wrong way.  I was thinking that the Logical Units and the Positions both had to do with the hard drive, but maybe this excerpt from the MTCopy manual sheds some light on this.

Point 4 MTCopy User Manual Rev 2a - page 14

Interesting how it says "Each tape in your set of backups will contain the bootable portion followed by the individual logical units."

So, maybe the Logical Units represent the partitions on the hard drive, but the positions represent the position that the Logical Unit is now holding on the QIC tape.  Could this be it?  I'm starting to think so.

I wondered why LU 0, 1 and 2, corresponded with Pos 1, 2 and 3, but LU 3 and 4 corresponded again with pos 1 and 2 instead of 4 and 5, but if the position means the position on the tape, and has nothing to do with the position on the hard drive, then this makes sense.  LU 3 is the first file after the BOOT file on the 2nd tape, so it is pos 1.

Mystery solved...I hope...