On TSA’s website they describe the pat-down screening as a procedure which is used to “determine whether prohibited items or other threats to transportation security are concealed on the person”.
“You may be required to undergo a pat-down procedure if the screening technology alarms, as part of unpredictable security measures, for enhanced screening, or as an alternative to other types of screening, such as advanced imaging technology screening. Even passengers who normally receive expedited screening, such as TSA Pre✓® passengers, may at times receive a pat-down,” they state.
TSA adds that their pat-down procedure includes inspection of the head, neck, arms, torso, legs and feet.
“This includes head coverings and sensitive areas such as breasts, groin, and the buttocks. You may be required to adjust clothing during the pat-down.