Also called: relay replication, replication tiers
In plain English
Cascading replication is when a standby streams WAL onward to other standbys instead of every standby connecting directly to the primary. The intermediate standby runs its own walsender and relays WAL downstream, forming a tree of replicas (primary → tier-1 standby → tier-2 standbys).
Why it matters
It offloads walsender fan-out from the primary and collapses redundant cross-region traffic — one WAL stream crosses a WAN link and fans out locally. The catch: cascaded standbys can’t be synchronous, since the primary only negotiates durability with its direct children.