What a visa is
A visa (or visa exemption) is about whether your nationality may travel to Thailand for a given purpose and stay length. Some travelers need to apply for a visa in advance. Others can enter without one under exemption rules. That is a separate process from TDAC.
What TDAC is
TDAC is an arrival card — a digital form with your identity and trip details. Most travelers complete it even when they do not need a visa. It does not replace a visa if your passport requires one.
You may need one, the other, or both
Visa-exempt visitors still usually need a TDAC. Travelers with a visa still usually need a TDAC. Neither document is a promise that you will be admitted. The officer at the border decides.
TDAC Entry only helps with the arrival card. We do not issue visas and we do not decide entry. The official TDAC form is free if you want to file it yourself.