Orders of battle
From Halopedia, the Halo wiki
Orders of battle are a concept employed by the Ecumene, used for classifying and categorising Forerunner spacecraft. Orders of battle are determined based on a given ship's offensive and defensive power, alongside ancillary capabilities they may additionally posess - though the evolution of strategy and technology over time mean that order ratings were constantly in flux throughout the lifespan of the Forerunners' empire. Additionally, while ratings were assigned relative to ships in the same category as 
Order ratings are assigned based on the relative power of a ship within its class. As such, a given ship type (such as dragoons) may have ship classes of multiple different orders; all rated relative to each other. A primary warship hypothetically rated in a lower order may still be individually more powerful than a higher-order escort or auxiliary ship. On top of the order rating and ship type, a ship may also then be given a more traditional classification based not on the systems the ship possesses, but rather its formative nature during the era it was constructed and as it accorded to the tenets of the Mantle - the Forerunners' guiding philosophy. These class names can manifest themselves in numerous ways, including (but not limited to);
- The ship's power or size (eg. Gargantua or Fortress-classes)
- The ship's speed (eg. Harriers or the Falco)
- The overall cultural shifts occuring during the era in which the ship was created (eg. Despair or Dignity-classes)
- The decisive goal of those who originally comissioned the ship as to embody a moral standard invoked by the Mantle (eg. Preservation or Sojourner-classes)
To give a worked example, consider the Sojourner-class dreadnought. In full, the Sojourner-class is considered a Prime Dragoon. This can be broken down as follows;
- Prime: A prime warship is considered a first-order warship; these represent the apex of military power within a given Forerunner ship classification. However, second and third (etc) order dragoons do exist.
- Dragoon: Dragoons represent the purest form of warship available to the Forerunner fleet. They were fast and heavily armed, though the ship type does not necessarily imply much more about the ship's combat roles. Many types of ship can be classed as dragoons including cruisers, battlecruisers and dreadnoughts.
- Sojourner: The name of the ship class's class, determined based on the factors listed above.
- Dreadnought: The ship's classification, and the role it plays out on the battlefield.
At least twelve orders of battle were considered by the Forerunners. On the largest and most powerful end were the prime dragoons, the most powerful regular warships available to the Ecumene ranging thousands or even tens of thousands of kilometres in length. On the smallest known end are the escorts, the twelfth order of which measured only a few dozen metres in length and served as a generalist utility escort ship and shuttle craft. Auxiliaries and interdictors were generally rated as mere sixth- or seventh-order hulls. The unique command ship Mantle's Approach was considered an unordered vessel.
Note that while Phaetons are referred to as Tertiary weapon-ships, this does not appear to be the same classification system as the orders of battle. Likewise, the peacemakers employ their own Tiered method of classification independent of the orders of battle.