The traditional core of CSCW focuses on the relationships, tensions, and gaps between technical systems and social activity. Policy orbits around this core as a persistent but marginally represented presence. In the last few years, however, CSCW has witnessed an upsurge of interest in (re)integrating policy more explicitly and meaningfully into research and practice. For example, recent scholarship stressed the mutually constitutive and interconnected threads of design, practice, and policy [31]. This paper expands upon those motivations through a qualitative case study of the role of policy in library mass digitization work and the subsequent emergence and evolution of the HathiTrust cooperative partnership. By tracing the origins and impacts of early policy decisions in this context, this research contributes to understandings of how and why policy can both close and open spaces of social practice and technical design, functioning as a source of embedded generativity in complex sociotechnical systems.