I use the Grouping id3 tag in iTunes to add subcategories to music where I have large numbers of albums, or where subcategories can be used across major categories (in my library, the subcategory grouping “Metal” shows up under Rock, Jazz and Progressive genres). But Plex neither displays or allows me to search the Grouping tag. Does anyone have a solution? If I knew what mp3 tags Plex does search, I could copy that tag over en masse.
I think there’s no way to actually use the existing tags… however - contrary to iTunes, Plex supports assignment of multiple Genres and Moods. So how about using that instead of maintaining your iTunes workaround? 
It’s a good suggestion, but since this is not tag-based this would mean a lot of effort categorizing music in the Plex database. I tried to avoid doing that in iTunes for the same reason: the (bazillion) files would have info in them that would fail to port to a different playback platform in the future–and wouldn’t even survive a reset of the Plex database if I needed to do that at some point.
