DAC6 concerns the spontaneous exchange of information on potentially aggressive tax arrangements. With the implementation of DAC6 into the national laws of the Member States comes a lot of uncertainty, along with diverging interpretations among Member States. In this article, the authors analyze the autonomy of Member States in the definition and interpretation of the concepts used in DAC6. The authors also analyze the relevant sources of the interpretation of DAC6, such as the relevant BEPS reports. The authors argue that DAC6 lays down a uniform framework for the spontaneous exchange of information of potentially aggressive tax arrangements. Member States do not have a margin of discretion regarding the interpretation of the concepts that are essential to the uniform framework. Other concepts may leave a margin of discretion for the Member States, such as several concepts used in the Hallmarks. In their margin of discretion, the Member States must ensure the full effectiveness of Union law. On the basis of that, the concepts must be defined (and interpreted) in line with the object and purpose of the Directive. Member States should use BEPS Action 12 as a source of interpretation and illustration insofar DAC6 is based on this report.