Many projections of near-future electricity system foresee a constantly increasing necessity of power flexibility services. In particular, thanks to the growing presence of renewable generation and innovative load technologies, distribution resources are becoming attractive products in ancillary services markets. In order to open the market gates to distribution flexibility, constant interactions between transmission and distribution system operators are required and the European project SmartNet is investigating in detail the possible coordination schemes among these two actors. The paper describes the SmartNet simulator, one of the main tools developed within the project, which precisely estimates the impact of TSO-DSO coordination schemes from the bidding and market clearing perspective, taking into account the consequent effects on the network physics at both transmission and distribution level.