Recommendation ITU-T E.164 provides the number structure and functionality for the five categories of numbers used for international public telecommunication: geographic areas, global services, Networks, groups of countries (GoC) and resources for trials. For each of the categories, it details the components of the numbering structure and the digit analysis required to successfully route the calls. Annex A provides additional information on the structure and function of international public telecommunication numbers (hereafter referred to as "international ITU-T E.164-numbers"). Annex B provides information on network identification, service parameters, calling/connected line identity, dialling procedures and addressing for geographic-based ISDN calls. Specific ITU-T E.164-based applications, which differ in usage, are defined in separate ITU-T Recommendations.