Postgres replication slots utilize two log sequence numbers (LSNs) — confirmed_flush_lsn and restart_lsn — to manage data streaming and retention effectively. The confirmed_flush_lsn indicates the last acknowledged data by the consumer, while the restart_lsn serves as a retention boundary for WAL segments needed for ongoing transactions. Understanding these differences is essential for troubleshooting replication issues and optimizing WAL retention in production environments.