Still don't understand the connection between "Fed" and "Mastodon". I think a Mastodon-based social network should be called "Tuskverse" or "Pachyverse" or "Stompverse"
Being based on the same protocol, they are all to some extent interoperable - e.g. you can follow Pixelfed users from your Mastodon account.
"Fediverse" is the umbrella term for all of these applications working together. It is a portmanteau for "federated" and "universe". The "federal police" association is unfortunate, but it is something USA only - rest of the world doesn't really see this.
The US holds no monopoly on the concept of a federation. Germany (Föderale Bundesrepublik) is one too, so is India or Russia.
It's the first time I hear someone take offense with the name. I think it's nice honestly, captures the essence of the network well.