Answer:
If they believe they may have a weapon.
Step-by-step explanation:
Stop and frisk policy allows individuals to be briefly but discreetly detained by the police. The police will do this if they believe the individual may be engaging in illegal behavior or activities. (Stealing, selling illegal drugs etc)
They might patdown individuals exterior clothing if they believe the individual may be hiding a weapon.
To patdown means to touch you from your top or shirt down to your pants, in such a way that if you are hiding any hard object like a gun or a knife it will be detected.