This ETS describes the general aspects on how supplementary services in the GSM system are realized from a technical point of view. Description of technical realization for specific supplementary services can be found in GSM 03.8x and 03.9x-series technical specifications. All supplementary services may require signalling on the radio path. Signalling procedures and messages used are defined in the GSM 04.8x and 04.9x-series of technical specifications. For some supplementary services information needs to be transferred between the home location register (HLR), the visitor location register (VLR) and the mobile services switching centre (MSC). Signalling procedures for such information transfer are defined in technical specification GSM 09.02. Definitions and descriptions of supplementary services are given in the GSM 02.8x and 02.9x-series of technical specifications. Definitions are given in technical specification GSM 02.04. NOTE: The technical specifications on the technical realization of supplementary services do not distinguish between subscriber, user and customer, since all three do not fully cover the textual needs. Generally the term subscriber is used, even if this person is not having the subscription.