Why do I need to provide documentation when ordering a certificate?
Additional validation steps, like paperwork during certificate issuance, are required only for the Organization validation and Extended validation SSL certificates. This way you will obtain the certificate with a Site Seal that will be able to display information about your Company/ Organization. Additionally, with EV certificates it will be possible to indicate your Company’s name in the address bar of the browser.
That is why you will need to provide some documents issued by your local or state government showing that your business or organization is authorized to do business in your location.
Articles What do I need to validate an OV certificate? and What do I need to validate an EV certificate? will help to understand the validation process better and find a list of documents required for the authentication process.