diff --git a/core/lib/Drupal/Core/Cache/CacheItem.php b/core/lib/Drupal/Core/Cache/CacheItem.php
index a58c1fd..d74c0f6 100644
--- a/core/lib/Drupal/Core/Cache/CacheItem.php
+++ b/core/lib/Drupal/Core/Cache/CacheItem.php
@@ -10,6 +10,10 @@
 /**
  * Provides a default cache item.
  *
+ * This class extends \stdClass to retain backwards compatibility in the 8.x.x
+ * branch. The CacheItem has no private properties either as to mimic the
+ * behaviour of a stdClass object. This should be changed to be a real value
+ * object when backwards compatibility breaks are allowed (i.e. 9.0.x).
  */
 class CacheItem extends \stdClass {