Both MSSQL and Sybase has a replication feature, this means in practice that all servers have a real-time copy of the same database. In theory you can have an unlimited number of servers connected to the same replication retention policy.
The SBN client is connected to all servers at once, if a server is down due to maintenance or failure, it will immediately switch server without users/operators noticing it. This allows site administrators to windows/SBN update servers or replace hardware without downtime.
Users can also share workload across servers, running reports on the third server etc.