Pre-emphasis is a rarely used feature found on some CDs. It was most common on early 80s Japanese CDs.

https://wiki.hydrogenaud.io/index.php?title=Pre-emphasis

At the moment, I make note of this in the annotation field. I think it would be better if we ha a dedicated option for this next to "disc has a pre-gap track" and "disc contains data tracks" on the release tracklist editing page so that tools like Picard could automatically give tracks on a release a pre-emphasis flag. There are some music players and foobar plugins that will apply the correct EQ curve to a track without applying a permanent de-emphasis process, but this only works if the CD is properly tagged to do so. This would save some manual tagging work and would make it easier to search for pre-emphasis CDs (some people intentionally look for them because they're unusual).