The Unitarian Universalist Congregation is an ephemeral thing. Like Love, it can only exist as part of a relationship between people. Those people can fill out paperwork that allows them to jointly own a building or to open a bank account, but the congregation will always be made up of their relationships. To be a Unitarian Universalist congregation, those relationships have to have a purpose, whether you call it a vision, a mission, or just a covenant. That purpose, ultimately, have to be about making the world around you better through your combined effort and resources. That is part of the promise a congregation makes when it joins the Association.
The Unitarian Universalist Association is a tool created by the relationship between congregations that serves those congregations in fulfilling their mission. Likewise, the congregations are a tool created by the relationship between congregants in order to serve the world. The function of a congregation is to equip the congregants with the mental, emotional, and physical tools they need to go out in to the world and meet the needs of a world in pain, and love it until it stops hurting. Ours is a relational religion, built on our desire to build a better world for everyone. And we do it together.