diff --git a/api/envoy/config/core/v3/base.proto b/api/envoy/config/core/v3/base.proto
index 4dda93414660c..15a17b49384df 100644
--- a/api/envoy/config/core/v3/base.proto
+++ b/api/envoy/config/core/v3/base.proto
@@ -293,7 +293,7 @@ message HeaderValueOption {
   HeaderValue header = 1 [(validate.rules).message = {required: true}];
 
   // Should the value be appended? If true (default), the value is appended to
-  // existing values.
+  // existing values. Otherwise it replaces any existing values.
   google.protobuf.BoolValue append = 2;
 }
 
diff --git a/api/envoy/config/core/v4alpha/base.proto b/api/envoy/config/core/v4alpha/base.proto
index 0fc074a016c7d..b13b4e89bfd11 100644
--- a/api/envoy/config/core/v4alpha/base.proto
+++ b/api/envoy/config/core/v4alpha/base.proto
@@ -285,7 +285,7 @@ message HeaderValueOption {
   HeaderValue header = 1 [(validate.rules).message = {required: true}];
 
   // Should the value be appended? If true (default), the value is appended to
-  // existing values.
+  // existing values. Otherwise it replaces any existing values.
   google.protobuf.BoolValue append = 2;
 }
 
diff --git a/generated_api_shadow/envoy/config/core/v3/base.proto b/generated_api_shadow/envoy/config/core/v3/base.proto
index 56436381dfbf8..472b6b7d91d30 100644
--- a/generated_api_shadow/envoy/config/core/v3/base.proto
+++ b/generated_api_shadow/envoy/config/core/v3/base.proto
@@ -291,7 +291,7 @@ message HeaderValueOption {
   HeaderValue header = 1 [(validate.rules).message = {required: true}];
 
   // Should the value be appended? If true (default), the value is appended to
-  // existing values.
+  // existing values. Otherwise it replaces any existing values.
   google.protobuf.BoolValue append = 2;
 }
 
diff --git a/generated_api_shadow/envoy/config/core/v4alpha/base.proto b/generated_api_shadow/envoy/config/core/v4alpha/base.proto
index d5c6062b5556f..af53aa402a472 100644
--- a/generated_api_shadow/envoy/config/core/v4alpha/base.proto
+++ b/generated_api_shadow/envoy/config/core/v4alpha/base.proto
@@ -292,7 +292,7 @@ message HeaderValueOption {
   HeaderValue header = 1 [(validate.rules).message = {required: true}];
 
   // Should the value be appended? If true (default), the value is appended to
-  // existing values.
+  // existing values. Otherwise it replaces any existing values.
   google.protobuf.BoolValue append = 2;
 }
 
diff --git a/source/common/router/header_formatter.h b/source/common/router/header_formatter.h
index 847657dba0d7c..65a996a5a9ebf 100644
--- a/source/common/router/header_formatter.h
+++ b/source/common/router/header_formatter.h
@@ -23,7 +23,7 @@ class HeaderFormatter {
 
   /**
    * @return bool indicating whether the formatted header should be appended to the existing
-   *              headers
+   *              headers or replace any existing values for the header
    */
   virtual bool append() const PURE;
 };
diff --git a/test/common/router/header_formatter_test.cc b/test/common/router/header_formatter_test.cc
index 8e0bfb20f3e59..93f626fc86e2d 100644
--- a/test/common/router/header_formatter_test.cc
+++ b/test/common/router/header_formatter_test.cc
@@ -957,7 +957,7 @@ TEST(HeaderParserTest, TestParseInternal) {
 }
 
 TEST(HeaderParserTest, EvaluateHeaders) {
-  const std::string ymal = R"EOF(
+  const std::string yaml = R"EOF(
 match: { prefix: "/new_endpoint" }
 route:
   cluster: "www2"
@@ -974,7 +974,7 @@ match: { prefix: "/new_endpoint" }
 )EOF";
 
   HeaderParserPtr req_header_parser =
-      HeaderParser::configure(parseRouteFromV3Yaml(ymal).request_headers_to_add());
+      HeaderParser::configure(parseRouteFromV3Yaml(yaml).request_headers_to_add());
   Http::TestRequestHeaderMapImpl header_map{{":method", "POST"}};
   NiceMock<Envoy::StreamInfo::MockStreamInfo> stream_info;
   req_header_parser->evaluateHeaders(header_map, stream_info);
@@ -983,7 +983,7 @@ match: { prefix: "/new_endpoint" }
 }
 
 TEST(HeaderParserTest, EvaluateEmptyHeaders) {
-  const std::string ymal = R"EOF(
+  const std::string yaml = R"EOF(
 match: { prefix: "/new_endpoint" }
 route:
   cluster: "www2"
@@ -996,7 +996,7 @@ match: { prefix: "/new_endpoint" }
 )EOF";
 
   HeaderParserPtr req_header_parser =
-      HeaderParser::configure(parseRouteFromV3Yaml(ymal).request_headers_to_add());
+      HeaderParser::configure(parseRouteFromV3Yaml(yaml).request_headers_to_add());
   Http::TestRequestHeaderMapImpl header_map{{":method", "POST"}};
   std::shared_ptr<NiceMock<Envoy::Upstream::MockHostDescription>> host(
       new NiceMock<Envoy::Upstream::MockHostDescription>());
@@ -1009,7 +1009,7 @@ match: { prefix: "/new_endpoint" }
 }
 
 TEST(HeaderParserTest, EvaluateStaticHeaders) {
-  const std::string ymal = R"EOF(
+  const std::string yaml = R"EOF(
 match: { prefix: "/new_endpoint" }
 route:
   cluster: "www2"
@@ -1022,7 +1022,7 @@ match: { prefix: "/new_endpoint" }
 )EOF";
 
   HeaderParserPtr req_header_parser =
-      HeaderParser::configure(parseRouteFromV3Yaml(ymal).request_headers_to_add());
+      HeaderParser::configure(parseRouteFromV3Yaml(yaml).request_headers_to_add());
   Http::TestRequestHeaderMapImpl header_map{{":method", "POST"}};
   NiceMock<Envoy::StreamInfo::MockStreamInfo> stream_info;
   req_header_parser->evaluateHeaders(header_map, stream_info);
@@ -1132,7 +1132,7 @@ request_headers_to_remove: ["x-nope"]
 }
 
 TEST(HeaderParserTest, EvaluateHeadersWithAppendFalse) {
-  const std::string ymal = R"EOF(
+  const std::string yaml = R"EOF(
 match: { prefix: "/new_endpoint" }
 route:
   cluster: "www2"
@@ -1161,7 +1161,7 @@ match: { prefix: "/new_endpoint" }
 )EOF";
 
   // Disable append mode.
-  envoy::config::route::v3::Route route = parseRouteFromV3Yaml(ymal);
+  envoy::config::route::v3::Route route = parseRouteFromV3Yaml(yaml);
   route.mutable_request_headers_to_add(0)->mutable_append()->set_value(false);
   route.mutable_request_headers_to_add(1)->mutable_append()->set_value(false);
   route.mutable_request_headers_to_add(2)->mutable_append()->set_value(false);