The TSA is intentionally vague about this. Via https://www.tsa.gov/travel/security-screening/identification

> The TSA officer may ask you to complete an identity verification process which includes collecting information such as your name and current address to confirm your identity. If your identity is confirmed, you will be allowed to enter the screening checkpoint, where you may be subject to additional screening.

> You will not be allowed to enter the security checkpoint if you choose to not provide acceptable identification, you decline to cooperate with the identity verification process, or your identity cannot be confirmed.

Every time I’ve flown for the past few years, my ID fails to scan at TSA. Don’t know why but every time it’s happened, the TSA agent will just call over another TSA rep, they will look at it, and then say I’m good to go ahead. Never knew these other methods existed, I’ve never been asked to confirm my identity a different way.