The charging supplementary service is described in GSM 02.86 [2] and t he relevant charging principles are given in the relevant GSM MoU Asso ciation documents. These services are designed to supply to a mobile u ser 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 Mobi le Station (MS) subscriber. In the case of certain MS uses, for example a mobile payphone, this es timate could be subject to further processing (e. g. to present the cha rges in currency, rather than units, this may include an additional ma rk up). This additional processing is not described in this ETS in ord er to avoid constraining the evolution of the MS product in this area. This European Telecommunication Standard ETS gives an overall view of how this supplementary service shall operate both in the PLMN and with in the MS. Text given in this ETS is required to define functionality and is not intended to constrain implementation.