In response to the rising demand for digital twins, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) launched Project PLATEAU, a 3D city model initiative. Japan has over 1,700 municipalities, each with unique characteristics that may lead to diverse 3D city models. However, differing specifications may compromise data reusability.<br />This paper presents the "Standard Data Product Specification for 3D City Models" (SDPS-3DCM), designed to facilitate the development, utilization, and availability of 3D city models. The SDPS-3DCM adopts CityGML and its Urban Planning Application Domain Extension (i-UR) as its application schema. We provided feedback to i-UR based on city use cases regarding missing features and attributes. Incorporating the updated i-UR improved the comprehensiveness of the SDPS-3DCM. Additionally, the SDPS-D3CM defines the subdivisions of Level of Detail (LOD) to promote a common understanding among data creators and users, increasing production efficiency and ensuring consistency. Each city can localize its specification by profiling the SDPS-3DCM, resulting in a 3D city model grounded in a unified specification while allowing for specific contextual variations. By 2024, over 200 cities developed 3D city models compliant with the SDPS-3DCM and made them publicly available as open data. The SDPS-3DCM not only accelerates the rapid development of 3D city models but also streamlines the creation and utilization of related tools. The SDPS-3DCM is expected to standardize 3D city models, address societal issues, and create new value across public and private sectors.