It means that you can choose where you want to send your notes and where you want to receive your replies, reactions, etc. On the other side, clients request notes to show followers and sends replies to where it's told, in order to be seen by you. You can totally do it now. It just requires using one of the clients that works that way. (And, you would want to know what it means, so you can choose relays that best align with your desired experience. )