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.