This science fiction series is set in the multiverse. It includes many parallel universes, about a half billion of them. Many universes are as large as the one we live in. Which has been expanding since the Big Bang, and has perhaps 200 billion galaxies. Each of which may contain as few as 10 million and as many as a 100 trillion stars.
Many are very similar to ours, with analogous histories. But many are tiny. Some have physical laws very different from our own.
Does this mean there are infinite variations of possibility? No. There is a quantum-mechanical order to the multiverse. Historical “world lines” (really “universe lines”) are each separated by some moderate distance from the next. There are no large world lines separated from it nearest neighbors by only a few atoms or by histories which have only a few nearly-indistinguishable events.
The first stories take place in a sheaf of universes all of which contain a Human Interstellar Confederation. Some of them contain world lines which have shapechangers in them. In short, Multiverse > Confed > Changers, where “>” means “contains.”