Levels of aggression can be found under a player's energy bar in the top left hand corner. City Guards will shoot anyone who has aggression within a radius around them for a high amount of damage, aggression also allows for players to hit other players with the same level of aggression in a No Fighting Zone (NFZ). Players without aggression in a NFZ are safe from being hit or slapped.
By default, a player's aggression is at 0% up until they swing and hit or slap and hit another user. After hitting or slapping a user once, aggression levels instantly rise to 100%, and will take 1 minute to drop to 0% again. Players with any aggression level are subject to getting hit or slapped by other players with aggression in No Fighting Zones. Aggression levels automatically decrease and will remain at 0% so long as a player is not in combat.
There are No Fighting Zones (NFZ) around the map, most notably all Corporations as well as rooms such as the Court, Clothing Store and Job Center. As long as you do not have aggression you cannot be damaged by any means in these rooms.