After the Dark War left millions dead, the need for crime regulation increased greatly. The crime rate had gone out of control due to the various problems that came from war, including discrimination between races and social classes, lack of food and shelter, and the influence of seithr.
Because the NOL trains elite soldiers who are very valuable, primarily operate as a large-scale military structure, tend to only get involved with things directly affecting the organization, and because they lack the manpower to form police forces, they established the vigilante system. It is a semi-regulated bounty hunting system in which the NOL rewards people who capture criminals, but takes no responsibility in the case of injuries or death. There are no tests or contracts required. The only requirements are that vigilantes are not allowed to organize or to involve civilians.
Crimes are reported to the NOL, who will either issue wanted posters of the suspect, or will dispatch vigilantes to investigate and find the perpetrators. Wanted posters include the name, face, and bounty of the criminal, and are distributed by the public offices of Hierarchical Cities, organizations responsible for managing civilians and helping citizens. It is extremely rare for the NOL to issue warnings along with bounties, as was the case with Ragna the Bloodedge. Bounties may require criminals to be brought in dead or alive, depending on the type of crime and the specifics of the case. The severity is not related to the criminal's rank.
Vigilantes use a special binding armagus taught by the NOL to capture criminals, and then report a special code to the NOL which allows them to recover the criminal, at which point the vigilante usually receives their reward. Completing bounties by other methods, such as reporting to the public office instead or bringing them in without a binding ars results in fees being subtracted from the reward. There's also a loosely organized network of vigilantes called the "Vigilantes Guild", which only exists to spread information between them.