How do I respond if additional documents are requested for my DTV application in Thailand?

The user manual, it starts on page 46 with how to see and how to upload when asked

If they request additional documents, it will normally show up inside your Thai e-Visa account/dashboard under your application status.

You’ll see something like “Request for Additional Documents” and be able to upload files directly there.

Sometimes you may also receive an email notification, but you usually respond by uploading through the e-Visa system, not by replying to email.

Additional document uploads will be through the evisa application portal.

They will send you an email asking for additional information and then they'll send you to a site to send it