Identity is not tied to a server; migration copies the data and updates the claim, friends do nothing, and a mirror keeps a continuous copy of your data (and your login data if asked).

Portability is the difference from federated systems.

Your identity is not tied to any server. The name, the keys, and the files are yours.

To move, you copy your data to a new home server.

Then you update your claim in the log: this is my home now.

No one you share with has to do anything. Links keep working. Friends stay friends.

A mirror does the same copy continuously. If your home goes down, the mirror has your data, and your login data if you asked it to.

Check yourself. What must your friends do when you move servers?