From what I understand from the official documents, PDS (Personal Data Servers) is actually very similar to relay, they are most likely to be normal servers, not personal devices. Posts are still sync in different PDS.

What's difference is：

1. on top of TPS/relay, there's a index crawler to handle events that's not in your closer social. (Section Achieving Scale),

2. User need to give PDS signing key to do anything with their events, which can be overriden by the user's Recover key. (Section Account Portability)

3. "Federation was chosen to ensure the network is convenient to use and reliably available." This is the biggest doubt I have after reading it, who choose which PDS to be included ?

But there is also DID documents store in personal devices, which you can broadcast to the other PDSs when the ones you are using is down. Check Account portability section. This is not in nostor protocol, but I think at least is working on it. (And I believe is a good idea the keep the protocol minimum and neat.)