Unclaimed Systems are those that have, for one reason or another, not been claimed by the UEE or any alien faction. There are various reasons for this.
1. Systems that have been claimed by organisations other than the UEE or Alien Nations, such as Outlaws/Pirates/Dissenters/Runaways etc.
2. Systems that have been deemed not worth developing by the UEE.
3. Systems that are not practical to access, or in one case possible to even find!
4. Frontier systems that simply haven't been explored yet.