diff --git a/changelogs/server_server/newsfragments/2159.clarification b/changelogs/server_server/newsfragments/2159.clarification
new file mode 100644
index 000000000..d7b79ccf5
--- /dev/null
+++ b/changelogs/server_server/newsfragments/2159.clarification
@@ -0,0 +1 @@
+Clarify the purpose of reference hashes.
diff --git a/specification/server_server_api.rst b/specification/server_server_api.rst
index 03c624abd..2e96dc858 100644
--- a/specification/server_server_api.rst
+++ b/specification/server_server_api.rst
@@ -1134,7 +1134,8 @@ Calculating the reference hash for an event
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The *reference hash* of an event covers the essential fields of an event,
-including content hashes. It is calculated as follows.
+including content hashes. It is used for event identifiers in room versions 3 and
+higher. It is calculated as follows.
 
 1. The event is put through the redaction algorithm.