Currently there is a primary relay that is manually selected with a checkmark or radio button in the interface. Your contacts will send messages to the relay they have seen you using the last time. Same for “Saved messages”, they are sent and received using the currently selected primary relay.
I also just wrote this howto with some screenshots, you can see how to select the default relay there:
In the future we want to automate this, e.g. do some failover in case your primary relay is unreachable, and send to multiple relays of the contact, but in 2.33 releases most of the time only one relay is used and non-primary relay is just to receive messages from contacts that don’t know that you have changed your primary relay yet.