This Global System for Mobile communications Technical Specification ( GTS) specifies the procedures used at the radio interface (reference p oint Um as defined in GSM 04.02) for normal operation, registration, e rasure, activation, deactivation, invocation and interrogation of comm unity of interest supplementary services. The provision and withdrawal of supplementary services is an administrative matter between the mob ile subscriber and the service provider and causes no signalling on th e radio interface. In GSM 04.10, the general aspects of the specification of supplementar y services at the layer 3 radio interface are given. GSM 04.80 specifies the formats coding for the supplementary services. Definitions and descriptions of supplementary services are given in GS M 02.04 and GSM 02.8x and GSM 02.9x-series. GSM 02.85 is related to th e community of interest supplementary services. Technical realization of supplementary services is described in techni cal specifications GSM 03.11 and GSM 03.8x and 03.9x-series. GSM 03.85 is related to the community of interest supplementary services. The procedures for Call Control, Mobility Management and Radio Resourc e management at the layer 3 radio interface are defined in GSM 04.07 a nd GSM 04.08. Signalling interworking for supplementary services between GSM 09.02 a nd GSM 04.08 and between GSM 09.02 and GSM 04.80 is defined in GSM 09. 11. The following supplementary services belong to the community of intere st supplementary services and are described in this GTS: - Closed User Group (CUG) clause 1.