The Outer Zone includes the Kuiper Belt, of which Pluto is the most famous (but not largest) body, and the Oort Cloud of distant long-period comets[...]
The Kuiper Belt, comprising the [Outer] zone, is perhaps the most mysterious of those parts of the Solar System since it is as yet unexplored by spacecraft. While there are more icy dwarf planets than in the other two zones, as yet no spacecraft has visited the region. This led the National Academy of Sciences to assign priority to missions to visit this third zone—especially the Pluto-Charon System — and the New Horizons mission was launched in 2006 to help fulfill this objective.
In the Kuiper Belt, icy dwarf planets represent a fascinating puzzle. Are they planetary embryos, whose growth was stunted? Are they relics of the origin of the solar system? Most scientists believe they are both, relics from the formation of the solar system more than 4 billion years ago. If they are the bodies out of which the larger planets accumulated, they promise new understandings of the origins and evolution of the solar system. With these efforts to explore the farthest zone of the solar system, humanity will complete the first stage of the reconnaissance of the Solar System. What is learned in the process will inform the direction of future explorations.
Source: https://launiusr.wordpress.com/2016/03/18/three-zones-of-the-solar-system/