The 'datum attribute' relationship on a 'classification datum' serves as an informal record of the significance of the (reified) classification. I.e., while any individual will be a member of several classes, the 'datum attribute' should point to a (shortcut) property that indicates the significance of the classification in question; this metadata is in general significant. For example, the 'datum attribute' can indicate that the classification establishes EX class, or the line size, of the subject.
