A SAD form is a customs form that is used in international trade. You will likely receive one of these when shipping from within the European Union to a country outside of the EU, or vice versa. It is also known as a C88 form.
You will need to make sure that it is properly filled in, and include it with your customs paperwork.
Note: The SAD is only used in situations where manual clearance is required, usually due to failed computer systems. Nevertheless, making sure it is correctly filled in is vital.