The Kryptonians categorized the technological advancement of civilizations into Classes. Every civilization the Kryptonians came into contact with were evaluated on certain behaviors as well as key technological developments. The higher the Class number, the more advanced the civilization's technology is/was.
Class One is one of the most common and earliest stage of civilization. Societies have no space-flight technology, or very limited space-flight, and tend to know little about other worlds or their inhabitants.
Earth is categorized into this level and humanity is difficultly adjusting to the existence of extraterrestrial life.
Class Two is which the species discovered space-flight, and possibly the existence of other species, but have yet to establish peaceful, cooperative relations with them.
Class Three is which societies have established peaceful relations with their interplanetary neighbours, and freely exchange information and conduct fair trade.
This level is where the Galactic Confederacy came to be.
Class Four is where societies have advanced even further. Societies are mostly peaceful, with almost none of the internal problems such as crime, pollution, and war. There tends to be an increased emphasis on science and technology, although this isn't essential for the society. Societies also do trade with other societies for resources, but not as much as others.
Class Five is essentially a continuation of the trend from Class Four. The only exception is their trade-needs are completely met, making them completely self-sufficient, and thus, do not carry out trade at all, or only in very rare circumstances, though they retain diplomatic relations with others.
Class Six is a special designation that is reserved for an entity, or society, capable of wiping out a Class Five society without contest. This level is incredibly rare, but they do exist.