RADIUS attribute from rfc4372


The Chargeable-User-Identity attribute, or CUI, (Type value 89) is a
   unique, temporary handle used as means to, for example, correlate
   authentication, accounting, and bill post-processing for a particular
   chargeable subscriber.  The CUI format and use follows guidelines
   defined by .

   In the scope of this document, the CUI attribute MAY be present in
   the Access-Request.  The CUI MAY also be present in the Access-
   Accept.  The CUI MUST be present in the Access-Accept if it was
   present in the Access-Request.  If the use of the Chargeable-User-
   Identity attribute is supported, then the MAG and/or the LMA commits
   to include the Chargeable-User-Identity attribute in all subsequent
   RADIUS Accounting packets they send for the given user.