RSI threads are asynchronous interfaces to send messages from one Replication Server to another. One RSI thread exists for each destination Replication Server to which the source database has a direct route.
The DIST thread in the primary Replication Server processes transactions, causing those destined for other Replication Servers to be written to RSI outbound queues. An RSI thread logs in to each replicate Replication Server and transfers messages from the stable queue to the replicate Replication Server.
When a direct route is created from one Replication Server to another, an RSI thread in the source Replication Server logs in to the replicate Replication Server. When an indirect route is created, Replication Server does not create a new stable queue and RSI thread. Instead, messages for indirect routes are handled by the RSI thread for the direct route. See Replication Server Administration Guide Volume 1 > Manage Routes.