现实中的自定义附庸（英文：Client state）是指一个在国际事务中，在经济层面，政治层面或军事层面上从属于另一个强国的国家。在游戏里，自定义附庸旨在模仿拿破仑时期法兰西共和国在邻国建立的一系列共和国，是为“姊妹共和国（英文：Sister republic）”。因此，玩家们有权创建自定义附庸来辅佐他们的征服大业。自定义附庸比其它附庸国更忠诚，其独立倾向-25。君主制自定义附庸从自定义开始便有着最大值的 正统性，共和制自定义附庸则拥有最大值的 共和传统。
Creating a client state
Once created, a client state behaves just like a vassal and is able to become a march. However, there is a maximum limit of 10 client states per nation. The ruler name for the client state is chosen from its culture as opposed to the dynasty of its overlord.
Since provinces owned by client states do not contribute to their overlord's overextension, they are a handy way to speed up expansion. By distributing newly acquired provinces to client states, the overlord can take far larger territories with a single bite, and annex them when the client states are done coring them.
Client states that share the culture of the provinces they rule avoid penalties to income and unrest for unaccepted cultures. If they share the religion of their overlord, they can handle the burden of converting their provinces, though they may not be able to if they don't take the Religious idea group and control provinces with high development.
When choosing which province the client state to make the capital, it's best to choose the province with the highest development, since the new client state will gain a free full core on that province and will not suffer overextension from that province. Although client states will not accept additional provinces that would put it over 100% overextension through the subject interaction tab, the player can manually give more provinces to the client state by clicking on the 'Create Client State' button a province adjacent to the existing client state and choosing the option to assign it to the existing client state.
It's also advisable to enable scutage on client states so that they are not called into wars and so can be annexed even if the overlord is at war.
The tags of created client states will start out as 'K00', followed by 'K01', 'K02' and so on.