Behind the innocuous name lies the national police force of Tian Xia. It has many roles, such as criminal investigation, counter intelligence, riot control, counter-terrorism, and hostage rescue. Different sections have different requirements for joining.
The largest by far, it has been split several times as various internal branches grow. It is primarily concerned with criminal investigations. Typically this section works with prefectural police to bring in criminals
Internal investigations. Charged with policing Public Security, which makes them somewhat unpopular amongst the rest of the organization. Also charged with investigations on prefectural police, which tends to make them unpopular with prefectural police.
Marshall section. Subordinate to the court system, and serve legal documents issued by the courts, as well as apprehend wanted fugitives, transport of prisoners, and operates the witness protection program
Technical operations. Conducts forensics and other technical testing (testing of weapons to determine tax category, etc)
Explosives Ordnance Disposal. A relatively small and some would say, crazy branch.
The direct action unit. This unit is specifically trained in small unit tactics, and would be largely ineffective in pitched battles. Fortunately, they have never had to.
Cybercrime. A relatively new branch.