There are some people who use federation and confederation interchangeably when this should not be the case. A confederation is a type of membership wherein the members can join voluntarily. For example, there is a confederation in a city. Those who would like to become part of the confederation regardless of their age, gender, social status, and so much more can join. In the federation, membership is not voluntary.
There are some people who are invited to become a part of the federation. Those who would like to join may send feelers so that they can be noticed by the other members of the federation, but if they are not invited, they cannot become part of it.
Federation and confederation do not exactly mean the same thing even though both words have similar spelling patterns. Federation refers to the act of joining together into a single political entity. In a federation, there is a strong central power that the majority of the power. It holds the sovereignty, and the member states have a certain level of autonomy. The membership of the federation is not voluntary.
This array of nations or states that are unified under one central authority select the members that control the central authority. Confederation, on the other hand, is a union or alliance of states or political organizations where members maintain their sovereignty and sometimes adopt a common constitution. Confederation is voluntary. The central power is weak compared to the power each member state has.