Technological Achievement Tiers
The Forerunners categorized the Technological Advancement of civilizations into Tiers. This system was later adopted by the Covenant. All sentient life and civilizations in the Universe fall into one of the categories. There are a total of seven tiers, and the lower the Tier number, the more advanced the civilization's technology is/was.
 Tier 7: Pre-Industrial
Tier 7 is one of the most common and stable states, with limited weaponry and environmental threats. Societies tend to be small and scattered, driven by subsistence farming, foraging, or hunter-gathering needs. Technology is limited to simple hand made tools, weapons, or agrarian implements and methods, but a very broad understanding of planetary and solar mechanics is not uncommon.
 Tier 6: Industrial Age
Tier 6 is often the pinnacle for a civilization. Agrarian societies can remain stable in the pre-industrial stage, but Tier 6 population strain and mechanized food production invariably create political and economic pressures very few can balance. Moving past this usually promises advancement. Some societies improve environmental and medical understanding concurrently with mechanical and transport advancement. Those that do not are frequently doomed.
This level is also where the Unggoy were before joining the Covenant. It is possible that the Jiralhanae stood in this level before joining the Covenant, because they had recently rediscovered the radio and flight before they were found by the Covenant.
 Tier 5: Atomic Age
Tier 5 species usually begin focusing on clean energy production. The occasional belligerent species will use atomic energy for weapons, often resulting in mass extinctions. In-atmosphere craft are a hallmark, often leading to manned space flight, albeit in a short-scale.
Humanity entered this age in 1945, when the first atomic bombs were dropped.
 Tier 4: Space Age
Tier 4 is often the final resting place for species intelligent enough to break free from their cradle's surface only to fill the gulf surrounding it with war. Their comfort-focused technology can include medical advances.
Humanity reached this level in 1957, when the first satellite was launched. This level is also where the Kig-yar and Yanme'e were before joining the Covenant. In some period of their history the Jiralhanae reached this level, but because of a bloody war between rival master-packs, their civilization fell back to the pre-industrial age.
 Tier 3: Space-Faring
Species has efficient slipspace navigation, mass drivers, asynchronous linear-induction weapons, holocrystal storage and semi-sentient AI (though their creation requires memory transfer from the freshly deceased and/or flash cloning). They have had no outside influence.
 Tier 2: Interstellar
The species has the ability to perform exceedingly accurate slipspace navigation, near-instantaneous interstellar communication and man-portable application of energy manipulation. The unified state of the 26th century Covenant stands at this level.
Both San 'Shyuum and Sangheili brought the other races of the Covenant to such a high technological level when incorporating them into the Covenant. The two primary extant interstellar civilizations were space-faring prior to discovering Forerunner artifacts - or each other - but they have not successfully reverse-engineered those artifacts.
Following the Human-Covenant War, humanity made use of captured Huragok and Forerunner artifacts to advance themselves. It is likely that they have reached Tier 2 by 2557. Notably, the UNSC Infinity is more advanced than any ship seen during the war, and humanity is known to make use of instantaneous interstellar communications as well as pinpoint slipspace navigation.
 Tier 1: World Builders
The species has the ability to manipulate gravitational forces, create AI with full sentience, fabricate super-dense materials, perform super-accurate slipspace navigation, the ability to create life, and the ability to create worlds. The Forerunners, prehistoric San 'Shyuum and prehistoric humans are the only known civilizations to reach this level.
 Tier 0: Transsentient
As the Forerunners had no examples of civilizations with technological accomplishments greater than themselves - with the exception of the Precursors - this is a theoretical ceiling. It is suspected that they can travel across galaxies and accelerate the evolution of intelligent life.
The technological achievement scale is similar to the real-world Kardashev scale, which measures technological advancement based upon the total amount of energy a civilization is capable of harnessing. Similar categorical scales are commonly found in science fiction.