The term trizonal is referring to the three regions of the Solar System. For decades the informal division of the Solar System was into the Inner and Outer Solar System, with the division point being the Asteroid Belt. In those days the Oort Cloud was theorized, but most people thought of the Solar System ending at the orbits of Neptune/Pluto.

The discovery of the Kuiper Belt pushed out the understanding to the volume of the Solar Systems. So with this concept inspired the idea of a hard science fiction setting set in the 22nd century with a system wide interplanetary civilization. So conceptually the Solar System is divided into three regions as humans move out into the Solar System.

Inner Solar System-from the Sun to the mean average orbit of the Asteroid Belt.
Middle Solar System-from the Asteroid Belt out to the orbit of Neptune.
Outer Solar System- from the orbit of Neptune to the Kuiper Cliff.

The Kuiper Cliff is an observation by astronomers that beyond the 50 AU there are few objects larger than 100 kilometers discovered. There are other objects beyond this Outer Zone, but these are so far out into the Oort Cloud as to not be easily useful for even a system whole spacefaring civilization.

Such a civilization would be defined by the travel times between the worlds. An interplanetary civilization would have a wide range of propulsion technologies for use. A number of these propulsion technologies would be in use at any one time. The missions and journey purposes would be vast and complex for a civilization spanning the Solar System, from robotic slow cargo ships up to rapid manned packet craft.