This document reports Network Digital Twin use cases in the IOWN Global Forum, providing an overview of the business motivations of such use cases, resulting in added values for various stakeholders such as network operators, network providers, and service providers. In addition, this document explains the relevance of IOWN Global Forum for the development of the Network Digital Twin. There are eight use cases described in the document, divided into two categories: Network Digital Twin for network management, and Network Digital Twin for vertical applications. Each use cases analyzes functional requirements, needed interfaces and data models, and pre-standardization analysis. Finally, this document analyzes the realization of the Digital Twin of networks through functional blocks identification on top of the IOWN technology infrastructure.