Kurt,
Position in Media-Set is used for multi-disc albums. I'll use an example that I posted on iPodlounge...
My album listing (both iPod and MC9's, but I'm more concerned about my iPod's) is hogged up by a listing like the one that follows:
Final Fantasy VII OST [CD 1]
Final Fantasy VII OST [CD 2]
Final Fantasy VII OST [CD 3]
Final Fantasy VII OST [CD 4]
Final Fantasy VIII OST [CD 1]
Final Fantasy VIII OST [CD 2]
Final Fantasy VIII OST [CD 3]
Final Fantasy VIII OST [CD 4]
For the Position in Media-Set tag, there's two values. it's basically "x of y," where x = current disc number and y = total disc numbers. The reason for having this is so that if whatever program you uses recognizes the tag, it lists the tracks in proper order according to what disc they're in. For instance, in MC9 right now, if I were to change the Final Fantasy VII OST [CD 1], [CD 2], [CD 3], [CD 4] to just Final Fantasy VII OST.... the tracks would be displayed in this order: 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4,4, 4, 5, 5, 5, 5, etc. If the program interprets the tag properly, the tracks would be written like 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5. Additionally, the program should write to the iPod in this order, that way no unnecessary screen real estate is used and I'll be able to listen to an entire four-disc album without having to create a playlist for it or play by Artist (which can be an inconvenience as sometimes there are extra discs with that artist name).
I'm sure a lot of people would appreciate this feature, I know I saw one guy on iPodlounge talking about it. Funny thing is, I didn't even know about the Position in Media-Set tag until someone mentioned what it did and that it worked with iTunes on the iPodlounge forums. Right then, I wanted the ability to use it as well, considering the large amount of four-disc albums I have.
Anyway, peace!