They are just two different pieces of metadata that came along at different times that represent similar ideas, so we made it so that, by default, Photo Mechanic will update both Color class and XMP label simultaneously and from the user perspective treat them essentially as if they are the same thing. Color classes are a numeric value that only Photo Mechanic knows about. XMP labels are text strings that several applications will read and update. Unless you are experiencing an issue with them, there isn't much of a reason to stray from the default settings with respect to this metadata.
