Unclaimed systems are those that while they may be within the territory of a space-faring race, are not administered by any race, and typically have little or no law enforcement presence. Consequently, these unclaimed systems tend to be gathering points for those beings who avoid the law.
The UEE specifically, tends to flag such systems with cautionary warnings to avoid travel there if possible.
Unclaimed systems do not tend to have any government structure as such, though local War Lords or Pirate Clans may sometimes exert some measure of control over what happens within an Unclaimed system.
As there are no formal governments for Unclaimed Systems, there is no formal diplomacy. Individuals or groups may contact and transact business with beings or groups currently in charge of these systems, but they do so at their own risk.
Government As A Character
Since unclaimed systems tend to be mostly lawless, there really isn't much to build a character on. Specific individuals or groups that are strong enough to hold any amount of power over such systems are going to be ruthless, and rule with an iron fist. Think of any world dictator, and you have a good idea of the character type.
More likely, there will be more than one being or group, seeking to wrest control of the system from all others. Therein lies the conflict that is at the heart of most stories.
Primary article source: Writers Guide: Part Three.