Learning from data is the point of making and using them. But how can scientists pass on to potential users what they have learned about their data? This chapter proposes answers to this question by focusing on the medium of data and its social uses. It argues that scientific data do not merely represent information but can be structured and presented to have a pragmatic function oriented to enable users’ understanding. It demonstrates this by describing how the MUWAGS collaboration, discussed in Chapters 6 and 7, designed its catalog – a table of the measured and estimated properties of galaxies –, to guide users to self-correct wrong uses and delimit itself from being held accountable for misuses. Some astronomers argue that catalogs encode their makers’ collective knowledge of their data. This chapter examines this claim ethnographically, suggests elements of a pragmatics of data reuse, and ends with a reflection on socio-computational orders – entanglements of the social and the computational in scientific work with large datasets.