The embassy just wants to be sure you work legally and out of Thailand. I'd recommend invoices or receipts - or if you can't provide them - last year's tax declaration. Business registration is also handy.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.