The charging supplementary service is described in GSM 02.86 [2] and the relevant charging principles are given in the relevant GSM MoU Association documents. These services are designed to supply to a mobile user sufficient information to allow a real-time estimate to be made of the bill which will eventually be levied in the home PLMN on the Mobile Station (MS) subscriber. In the case of certain MS uses, for example a mobile payphone, this estimate could be subject to further processing (e.g. to present the charges in currency, rather than units, this may include an additional mark up). This additional processing is not described in the present document in order to avoid constraining the evolution of the MS product in this area. The present document gives an overall view of how this supplementary service shall operate both in the PLMN and within the MS. Text given in the present document is required to define functionality and is not intended to constrain implementation.