Was there an RFC that went on to define the proper usage of the Class attribute, or is it's usage still ambiguous ?
Ambiguous how? The RFC seems pretty specific to me; the field is NOT to be interpreted by the NAS, is generated in the Access-Accept and sent in Accounting-Request - i.e. "it's local to the radius server, do what you like with it".
I know some people use it to link accounting data to an authentication attempt....
That's one (common) use. There are others e.g. I've used it to signal to the accounting server the type of NAS so that the exact method for kicking a session is known purely from the SQL accounting info.
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