CBT – Computer Based Testing

arts of the USMLE are administered by computer. Prometric provides scheduling and test centers for the computer-based components of the USMLE. Step 1 and Step 2 CK are given around the world at Prometric Test Centers (PTCs). Step 3 is given at PTCs in the United States and its territories only.

Source: http://www.usmleworldwide.com/blog/?p=59