diff --git a/core/lib/Drupal/Core/Block/MainContentBlockPluginInterface.php b/core/lib/Drupal/Core/Block/MainContentBlockPluginInterface.php
index 55fc292..2516348 100644
--- a/core/lib/Drupal/Core/Block/MainContentBlockPluginInterface.php
+++ b/core/lib/Drupal/Core/Block/MainContentBlockPluginInterface.php
@@ -10,6 +10,8 @@
 /**
  * The interface for "main page content" blocks.
  *
+ * A main page content block represents the content returns by the controller.
+ *
  * @ingroup block_api
  */
 interface MainContentBlockPluginInterface extends BlockPluginInterface {
diff --git a/core/lib/Drupal/Core/Display/Annotation/PageDisplayVariant.php b/core/lib/Drupal/Core/Display/Annotation/PageDisplayVariant.php
index bc07297..f04024d 100644
--- a/core/lib/Drupal/Core/Display/Annotation/PageDisplayVariant.php
+++ b/core/lib/Drupal/Core/Display/Annotation/PageDisplayVariant.php
@@ -13,6 +13,9 @@
  * Page display variants are a specific type of display variant, intended to
  * render the main content of a page.
  *
+ * For example, the FullPageVariant is used by the Block module to control
+ * regions and output block content placed in those regions.
+ *
  * @see \Drupal\Core\Display\VariantInterface
  * @see \Drupal\Core\Display\PageVariantInterface
  * @see \Drupal\Core\Display\VariantBase