This is a very important question. Each embassy or consulate will have their own supporting document requirements. Some of the most common supporting documents are passport copies, cover letters, application forms, and photocopies of the authenticated documents. However, some less common supporting documents are certified translations and copies of the business registration (for business documents). These are document and/or country specific requirements that only apply if your destination country is not a signatory to the Hague Apostille Convention.
To find out what supporting documents you will need to provide, contact an IDC expert.