Here are the rules of thumb.
If your drone has a C1, C3 label: the A1-A3 certificate is required. If your drone has a C2 label: the A1-A3 certificate and A2 certificate are required.
If you fly a drone without a Cx-label and it weights more than 249 grams then the A1-A3 certificate is required. For drones with a C0 label or less than 250 grams, we recommend the A1-A3 certificate.
If you want to operate in the specific category (C5/C6 label), you will need the STS theory certificate.