diff --git a/core/lib/Drupal/Core/EventSubscriber/HtmlResponsePlaceholderStrategySubscriber.php b/core/lib/Drupal/Core/EventSubscriber/HtmlResponsePlaceholderStrategySubscriber.php
index 147d51f..ca7f2ac 100644
--- a/core/lib/Drupal/Core/EventSubscriber/HtmlResponsePlaceholderStrategySubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/HtmlResponsePlaceholderStrategySubscriber.php
@@ -14,7 +14,13 @@
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
- * HTML response subscriber to allow for different  placeholder strategies.
+ * HTML response subscriber to allow for different placeholder strategies.
+ *
+ * This allows core and contrib to coordinate how to render placeholders;
+ * e.g. an EsiRenderStrategy could replace the placeholders with ESI tags,
+ * while e.g. a BigPipeRenderStrategy could store the placeholders in a
+ * BigPipe service and render them after the main content has been sent to
+ * the client.
  */
 class HtmlResponsePlaceholderStrategySubscriber implements EventSubscriberInterface {
 
diff --git a/core/lib/Drupal/Core/Render/Placeholder/SingleFlushStrategy.php b/core/lib/Drupal/Core/Render/Placeholder/SingleFlushStrategy.php
index 61fea0f..d9f8545 100644
--- a/core/lib/Drupal/Core/Render/Placeholder/SingleFlushStrategy.php
+++ b/core/lib/Drupal/Core/Render/Placeholder/SingleFlushStrategy.php
@@ -11,7 +11,8 @@
  * Defines the 'single_flush' placeholder strategy.
  *
  * This is designed to be the fallback strategy, so should have the lowest
- * priority.
+ * priority. All placeholders that are not yet replaced at this point will be
+ * rendered as is and delivered directly.
  */
 class SingleFlushStrategy implements PlaceholderStrategyInterface {
 
diff --git a/core/tests/Drupal/Tests/Core/Render/Placeholder/PlaceholderStrategyManagerTest.php b/core/tests/Drupal/Tests/Core/Render/Placeholder/PlaceholderStrategyManagerTest.php
index 387cef5..87c766a 100644
--- a/core/tests/Drupal/Tests/Core/Render/Placeholder/PlaceholderStrategyManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/Placeholder/PlaceholderStrategyManagerTest.php
@@ -41,7 +41,7 @@ public function providerProcessPlaceholders() {
     $data = [];
 
     // Empty placeholders.
-    $data[] = [[], [], []];
+    $data['empty placeholders'] = [[], [], []];
 
     // Placeholder removing strategy.
     $placeholders = [
@@ -52,7 +52,7 @@ public function providerProcessPlaceholders() {
     $prophecy->processPlaceholders($placeholders)->willReturn([]);
     $dev_null_strategy = $prophecy->reveal();
 
-    $data[] = [[$dev_null_strategy], $placeholders, []];
+    $data['placeholder removing strategy'] = [[$dev_null_strategy], $placeholders, []];
 
     // Fake Single Flush strategy.
     $placeholders = [
@@ -63,7 +63,7 @@ public function providerProcessPlaceholders() {
     $prophecy->processPlaceholders($placeholders)->willReturn($placeholders);
     $single_flush_strategy = $prophecy->reveal();
 
-    $data[] = [[$single_flush_strategy], $placeholders, $placeholders];
+    $data['fake single flush strategy'] = [[$single_flush_strategy], $placeholders, $placeholders];
 
     // Fake ESI strategy.
     $placeholders = [
@@ -77,7 +77,7 @@ public function providerProcessPlaceholders() {
     $prophecy->processPlaceholders($placeholders)->willReturn($result);
     $esi_strategy = $prophecy->reveal();
 
-    $data[] = [[$esi_strategy], $placeholders, $result];
+    $data['fake esi strategy'] = [[$esi_strategy], $placeholders, $result];
 
     // ESI + SingleFlush strategy (ESI replaces all).
     $prophecy = $this->prophesize('\Drupal\Core\Render\Placeholder\PlaceholderStrategyInterface');
@@ -90,7 +90,7 @@ public function providerProcessPlaceholders() {
     $prophecy->processPlaceholders([])->shouldNotBeCalled();
     $single_flush_strategy = $prophecy->reveal();
 
-    $data[] = [[$esi_strategy, $single_flush_strategy], $placeholders, $result];
+    $data['fake esi and single_flush strategy - esi replaces all'] = [[$esi_strategy, $single_flush_strategy], $placeholders, $result];
 
     // ESI + SingleFlush strategy (mixed).
     $placeholders = [
@@ -118,7 +118,7 @@ public function providerProcessPlaceholders() {
     $prophecy->processPlaceholders($normal_result)->willReturn($normal_result);
     $single_flush_strategy = $prophecy->reveal();
 
-    $data[] = [[$esi_strategy, $single_flush_strategy], $placeholders, $result];
+    $data['fake esi and single_flush strategy - mixed'] = [[$esi_strategy, $single_flush_strategy], $placeholders, $result];
 
     return $data;
   }