http://2mu7qwudmq6he6mkvwysqwqkw5zhwecfayosftcauodzpfpkpb7rf5yd.onion/res/315.html
Open groups, unlike other services in Session, are self-hosted and thus not fully decentralised. Someone has to run a server which stores the open group's message history. Additionally, because open group servers can serve thousands of users, messages are only encrypted in transit to the server rather than being fully end-to-end encrypted.