It’s a “proof of life” form. No big deal. Just don’t ignore it. Send it back, or you’ll get your benefits stopped. You won’t lose them, but it will be a pain to get them going again.
What Stuart said. If you want to stay longer than the 90 days that the non-O visa allows and get a one year extension of stay, you’ll need that 800,000 baht in a Thai bank account 2 months before you file for it. Otherwise, you’ll be out of status 90 days after you enter the Kingdom and will have to leave and apply for another visa outside of Thailand or come back with a visa exempt for a 60 day stay.