Regulation (EU) 2019/947 is the implementing regulation on the rules and procedures for the operation of unmanned aircraft. It defines the three categories — open, specific and certified — and the open-category subcategories A1, A2 and A3.
It is the core "how you may fly" rulebook: pilot competency requirements, operator registration and geo-zones all come from it. In practice, this regulation stands behind every A1/A3 exam question, so it pays to understand its logic rather than memorise it.