45-12 is a numeric shorthand used by the Pluribus Police that indicates a civil disturbance with escalated violence.
While it potentially could involve any form of civil disturbance, as well as any form of what is considered to be "escalated violence" - at least according to information given to the dispatch officer, there are some locations that are famous among the local police for involving a certain type of disturbance combined with a certain type of escalated violence.