In the context of preliminary reference procedures, the Court of Justice of the European Union (CJEU) routinely clarifies national courts’ questions surrounding the application of union law in EU member states. In the process, the CJEU defines legal rules which guide the future decision-making of national courts adjudicating conflict between domestic and union law. What we know little about is how the CJEU decides on the appropriate level of specificity of these legal rules. Determinate rules promise to guide the future decision-making of national courts in cases with underlying facts very similar to the case at the heart of the preliminary reference procedure. Less specific rules promise to apply to a wider set of cases before national courts, albeit leaving national courts greater flexibility in interpreting the implications of the CJEU’s rules for their own actions. In this paper, I develop and test a formal model of the CJEU’s rule-making in preliminary reference procedures. The model identifies how existing precedents previously established by the CJEU as well as constraints faced by national courts when adjudicating conflict between domestic and union law shape the CJEU’s choice of rule specificity. The formal model’s empirical implications are tested using the text and metadata from the CJEU’s judgments in preliminary reference procedures between 1997 and 2018.