Allegiance is the measure of loyalty of a planet to an empire. In terms of game mechanics, allegiance determines an empire's economy.
The allegiance value of a planet is determined by two factors: the position of the empire's capital planet and culture.
The capital planet has 100% allegiance (with no culture upgrades); every planet farther from the capital planet will have a lower allegiance value. The position of planet in relative to the capital planet determines the maximum allegiance (for example, a planet one phase line away from the capital planet has 90% max. allegiance, a planet two phase lines away will have 80% max. allegiance, etc.). This pattern will continue until it reaches 35% allegiance and the only thing that can drop it below that is enemy culture.
Both friendly (domestic or allied) and enemy culture also affect allegiance. A dominant friendly culture will increase a planet's maximum allegiance, and a dominant enemy culture will decrease a planet's allegiance. A planet will, by default, reach its maximum allegiance, unless it is affected by enemy culture. It is not possible for an empire to use friendly culture to increase the allegiance of a planet once it has reached maximum allegiance.
The most common way is to put down a culture producing structure, the Broadcast Center, Temple of Communion, or the Media Hub. Culture alone will increase your allegiance by 10% on any world under its the effect of your culture.
With certain research (Allure of the Unity for Advent in Entrenchment) it is possible to have allegiance above 110% for your capital planet (up to 120%). Allure of the Unity gives you a 10% bonus maximum allegiance for all planets. The highest possible is 150% allegiance if you have a Starbase in your capital planet's gravity well and the Starbase is equipped with Induced Reverence (increases maximum allegiance by 15/30%). So it is possible for Advent players to have their farthest colonies to have 45% maximum allegiance (with culture) or 75% maximum allegiance with a Starbase with 2/2 Induced Reverence, which gives Advent players a big boost for late game economy in huge multi-system maps.
Effects of Allegiance
Planets with higher allegiance are more productive, generating more credit, metal, and crystal income. For example a capital planet with an allegiance of 100% will contribute 100% credit, metal, and crystal income to an empire, while a capital planet with 110% allegiance will contribute 110% income, an increase of 10%.
If the allegiance of a colony falls to zero, a revolt will occur and the colony will be lost to the empire it belonged to. The planet will become neutral and vulnerable to colonization by enemy empires. If the planet is under the influence of enemy culture it cannot be colonized by you or any allies until the enemies culture is repelled.