 This paper presents a theoretical market framework, TMF, for designing electricity markets that integrates TSO-DSO coordination mechanisms, outlines fundamental categories and decisions essential to market design, maps 13 European demonstrators of TSO-DSO coordination solutions, identifies real-world challenges in designing and implementing novel system services markets, and offers market design and policy recommendations to address these specific challenges. This article was authored by Matteo Trancia, José Pablo Chavez Avila, Carlos de Maas Silva, and others.