The Treaty of Farixen is a treaty signed by Council races limiting the number of dreadnoughts among the different Citadel races in their given fleets. At the Farixen Naval Conference, the Council races agreed to fix a ratio of dreadnought construction between themselves due to their destructive potential. At the top of the pyramid is the peacekeeping turian fleet which makes up most of the Citadel Fleet. Second, are the other Council races - the asari and the salarians. Council associate races, like the hanar and volus, are at the bottom of the list.
The ratio of turian to Council to associate dreadnoughts is 5:3:1, which essentially means for every five dreadnoughts the turians construct, the asari and salarians are allowed three, and all other Citadel races one. Signing the Treaty of Farixen is a requirement for any race wishing to open an embassy on the Citadel.
Carriers are megalithic ships that have a similar general design to that of Dreadnoughts. The only difference is that Carriers are designed to carry and transport mass amounts of vehicles, fighters, interceptors, and troops across the galaxy. Dreadnoughts are specifically designed to combat enemy ships in a space combat environment and/or bombard planetary targets. The construction of Carriers is not included in the Treaty of Farixen policy resulting in no legal limit upon building any number of Carriers for any race.
Mass Effect 2
If the Council was saved during the Battle of the Citadel, the Systems Alliance is given a seat on the Council out of gratitude for their efforts. This presumably means that, as a Council race, humanity is now entitled to construct three dreadnoughts for every five turian dreadnoughts.
If the Council was left to perish on the Destiny Ascension, news stories can be heard on the Citadel and Omega in reporting that the turians no longer feel bound by the Treaty, and are stepping up their shipbuilding efforts as a result.
Mass Effect 3
In 2186, just prior to the Reaper Invasion, an Alliance News Network interview" by Diana Allers reveals that after a brief conflict with the Blue Suns on Korlus, the Migrant Fleet had acquired salvage necessary to upgrade their ships in preparation for their upcoming war against the geth. They used the materials to arm every single ship in the Civilian Fleet, even the liveships, with Thanix cannons, essentially converting them into dreadnoughts, thereby violating the Treaty. When the Citadel condemned the Fleet's actions, Admiral Han'Gerrel vas Neema pointed out that since the Council closed down their embassy on the Citadel, the quarians are no longer bound by the Treaty and can do whatever they want with their ships.
Shepard can comment on this to Admiral Shala'Raan vas Tonbay, to which she replies that it isn't technically a violation, because liveships are specifically designed for food cultivation, not combat. When Shepard doubts that the Council will accept that claim, Raan replies that if she has to, she will formally apologize to them once the war has ended.
- The Treaty of Farixen is almost certainly based off the Washington Naval Treaty, signed by the U.S., Great Britain, Japan, France, and Italy in 1922, limiting these states to capital ship tonnage ratios of 5:5:3:1:1, respectively. It was an attempt at maintaining the balance of power and the peace after World War I. The treaty ended with the advent of World War II.
- An unintended consequence of the Washington Naval Treaty was an increase in development of aircraft carriers, which faced fewer restrictions than other vessels. There appears to be a parallel in Mass Effect in the development of carriers by the Systems Alliance. While carriers are as large as dreadnoughts, and their use in combat is seen as a major innovation, they are not restricted by the Treaty of Farixen.