"This standard defines the stage one of the subaddressing (SUB) supplementary service for the pan- European Integrated Services Digital Network (ISDN) as provided by European public telecommunications operators. Stage one is an overall service description from the user's point of view (see CCITT Recommendation I.130 [1])@ but does not deal with the details of the human interface itself. This standard defines the interworking requirements of private ISDNs with the public ISDN. In addition this standard specifies the base functionality where the service is provided to the user via a private ISDN. This standard does not specify the additional requirements where the service is provided to the user via a telecommunications network that is not an ISDN but does include interworking requirements of other networks with the public ISDN. Charging principles are outside the scope of this standard. Interactions with supplementary services not listed in Clause 8 are outside the scope of this standard. The Subaddressing (SUB) supplementary service allows the called (served) user to expand his addressing capacity beyond the one given by the ISDN number. The SUB supplementary service is applicable to all telecommunication services. This standard is applicable to the stage two and stage three standards for the ISDN SUB supplementary service. The terms ""stage two"" and ""stage three"" are also defined in CCITT Recommendation I.130 [1]. Where the text indicates the status of a requirement (i.e. as strict command or prohibition@ as authorisation leaving freedom@ or as a capability or possibility)@ this shall be reflected in the text of the relevant stage two and stage three standards. Furthermore@ conformance to this standard is met by conforming to the stage three standards with the field of application appropriate to the equipment being implemented. Therefore no method of testing is provided for this standard."