A subject empire is an Empire that has a special diplomatic relationship with another empire of superior strength. An empire can become a subject either forcefully through wargoals or willingly, though the latter requires both a superior relative power and a positive attitude towards the superior party. In the default galaxy map mode subjects have the same color as their overlord.
If they have fleets available, subject fleets tend to follow the overlord's.
There are a number of types of subjects available depending on the overlord's government. Most of them have to pay a certain tribute to the overlord.
For subject empires the attitude is replaced by loyalty, expressing how content they are living under their Overlord’s rule. If a subject becomes disloyal they refuse any deal with the overlord and have a chance to start a war for independence, always waiting for an opportunity when something depletes the overlord's relative power (a taxing war, a larger uprising, a crisis, etc.). The loyalty is determined by the subject's opinion of the Overlord, which aside from the normal factors is also affected by the total military strength of all the subjects relative to the Overlord as well as that of those who support their independence.
An Empire is able to support the independence of another Empire's vassal. Doing this will count the supporting empire's fleet for the relative power of subjects opinion modifier, causing the subject to rebel if they stand a good chance of victory. If they do, the Empire supporting them automatically joins their war for independence.
Subject empires do not suffer from border friction and can therefore in some cases be utilized as buffer states between two empires that otherwise would have significant border friction issues.
Vassals and Protectorates that an empire possess for longer than 3 600 days can be peacefully "integrated" by paying influence, at a rate of 5 influence a month. The base cost is 20, increased by 5 for each pop and 50 for each planet. Empires with the Shared Destiny ascension perk benefit from a -50% influence cost to integrating subjects. Only one subject can be integrated at a time.
Once the integration is completed, all planets and ships of the subject are transferred to the overlord; the ships and starbases retain their graphic type. All leaders, resources and armies are removed; new scientists are required for their science ships.
Releasing a vassal
A player can create a vassal from their own territory by releasing one of their sectors as a vassal from the "Sectors" tab by clicking the "Create Vassal" button next to the sector they want to release. The player can then transfer additional systems to their vassal through the Trade option "Transfer System" if they so choose. Sometimes it is preferable to conquer systems and then release them as a vassal since this will create a vassal that shares the overlord's ethics and government. Notably, the vassal will be granted all its overlord's technology. The option to release a vassal is not available to Determined Exterminators or for species with Culture Shock or the Hive-Minded and Nerve Stapled traits.
The Feudal Society Civic will make all of its Vassals more loyal to it as it causes all relation, opinion and trust penalties caused by relative fleet power to be cut in half. In addition to that, it also allows Vassals to build new Starbases and colonize new planets, something that is not possible otherwise. This opens up a lot of options for utilizing Vassals; One could, for example, create a Vassal by releasing a system you own as one and then delegating the task of colonizing worlds to the Vassal while the overlord focuses on their economy, fleet etc. Since the Feudal Society Civic greatly reduces opinion, relation and trust penalties incurred from relative fleet power, the vassal is a lot less likely to rebel and it can be integrated later to allow the overlord to claim the systems that the vassal has colonized.
Alternatively, when starting a new game, one could launch an early war to vassalize one or multiple of their starting neighbors. In the early stages of the game, most empires (aside from Advanced and Fallen Empires) are still in the process of establishing themselves and are thus likely going to be rather weak economically and militarily, making them relatively easy conquest targets for empires with enough military strength. This is where the Feudal Society Civic comes into play; without it, an empire vassalized early in the game is going to remain at largely the same power level it was at when it was vassalized, effectively stopping its expansion and development. As the game progresses and other empires grow bigger and stronger, empires vassalized early on in their expansion and development will quickly become powerless relative to their neighbors, making them of little use. If an empire has the Feudal Society Civic, however, any empire vassalized by it early in the game will still be permitted to build new Starbases to claim systems and colonize new worlds, allowing it to continue its expansion and development, becoming bigger and stronger along with its overlord. This can allow it to gradually become more powerful, granting a reasonably good mid-game ally for the overlord empire and once the vassal has served its purpose or if it holds valuable planets, it can be integrated by the overlord at their leisure as long as relations are good enough.
To make a similar situation without Feudal Society you can opt to have your subjects be a Tributary. While Tributaries are more independent than vassals and they can get into trouble with other empires, they offer minerals and energy rather than fleet size. Tributaries will expand their empire just like vassals with the Feudal Society Civic. Be aware that if they become a Tributary prior to having a shipyard they will not expand. You can either consider this a benefit if you want them to remain the same size, or make sure they have a shipyard or two somewhere in their space at the time they become a Tributary to expand their borders. Something to consider is creating a Tributary behind you, protected from other empires by making sure you control the only hyper-lanes into their pocket of the galaxy. This option is done by creating a vassal from a sector; Release the subjects; Demand they become a Tributary. They will usually accept, but if they deny you'll have Casus Belli on them, so a quick War and re-subjugating them as a Tributary can be a huge benefit early-mid game, with the option to become a full vassal (War may be required) for that much needed Navy size increase late game. If you can afford the influence, something to consider is to create a research agreement and/or commercial pact before demanding they become a Tributary. Keep in mind once they are Tributary neither you or they can break those two agreements (might be a bug or unintended/overlooked option). You can also create a migration treaty which can be broken after being subjugated.
Tributaries can be given systems and resources just like Vassals via the trade menu, but taking a system you want back will mean you need to have a war with them to do it, so be careful about what systems you give up. This alternative option to Feudal Society Civic gives your subjects the freedom to expand, and in MegaCorp expansion, the administrative overhead can be reduced by Tributaries but still get energy credit and mineral distributions each month. If you chose the research agreement and commercial pact before they became a Tributary you do collect their benefits along with the mineral and energy monthly tributes.