Here is the excerpt from the DHS document:
Individuals undergoing screening using AIT generally will have the option to decline an AIT screeningin favor of physical screening. Given the implementation of ATR and the mitigation of privacy issues associated with the individual image generated by previous versions of AIT not using ATR, and the need to respond to potential security threats, TSA will nonetheless mandate AIT screening for some passengers as warranted by security considerations in order to safeguard transportation security.