CTRU (Counter Terrorist Response Unit) was formed in Hong Kong in 2006, it was formed under Counter Terrorism and Internal Security (CTIS) Division, which is under B Department (Crime and Security), SDU is under Police Tactical Unit of A Department (Operational Bureau)
CTRU specialize in high risk operational target (Embassy, Financial and Security Centre) and was trained to provide security assistance and patrol of said High Risk Area, which its goal is to provide direct support and prevent threat. SDU and PTU is on the other hand is a direct action element, which deployed as a reaction force. While both CTRU and SDU are trained in CT Operation, CTRU is trained on other preventive measure (such as intelligence/information control, and aggressive patrol) SDU trained to deal with action directly related to the crisis, also SDU also highly specialized in Hostage Recovery. Which is one of the original goal for the unit.