diff --git a/openapi/openapiv2.json b/openapi/openapiv2.json
index c7fba397..12fe7040 100644
--- a/openapi/openapiv2.json
+++ b/openapi/openapiv2.json
@@ -4570,7 +4570,7 @@
         "version": {
           "type": "string",
           "format": "int64",
-          "title": "TODO: What is this? Appears unused by SDKs"
+          "description": "Namespace failover version. Defines alternate history branches that can arise in\nmulti-cluster configurations (e.g. multi-region namespaces)."
         },
         "taskId": {
           "type": "string",
diff --git a/openapi/openapiv3.yaml b/openapi/openapiv3.yaml
index d0e15256..05d4a504 100644
--- a/openapi/openapiv3.yaml
+++ b/openapi/openapiv3.yaml
@@ -2965,7 +2965,9 @@ components:
           format: enum
         version:
           type: string
-          description: 'TODO: What is this? Appears unused by SDKs'
+          description: |-
+            Namespace failover version. Defines alternate history branches that can arise in
+             multi-cluster configurations (e.g. multi-region namespaces).
         taskId:
           type: string
           description: 'TODO: What is this? Appears unused by SDKs'
diff --git a/temporal/api/history/v1/message.proto b/temporal/api/history/v1/message.proto
index da795e16..1972be4e 100644
--- a/temporal/api/history/v1/message.proto
+++ b/temporal/api/history/v1/message.proto
@@ -767,7 +767,8 @@ message HistoryEvent {
     int64 event_id = 1;
     google.protobuf.Timestamp event_time = 2;
     temporal.api.enums.v1.EventType event_type = 3;
-    // TODO: What is this? Appears unused by SDKs
+    // Namespace failover version. Defines alternate history branches that can arise in
+    // multi-cluster configurations (e.g. multi-region namespaces).
     int64 version = 4;
     // TODO: What is this? Appears unused by SDKs
     int64 task_id = 5;