diff --git a/core/lib/Drupal/Core/TypedData/DataDefinitionInterface.php b/core/lib/Drupal/Core/TypedData/DataDefinitionInterface.php
index cc8e03ceb3..2a8e89255f 100644
--- a/core/lib/Drupal/Core/TypedData/DataDefinitionInterface.php
+++ b/core/lib/Drupal/Core/TypedData/DataDefinitionInterface.php
@@ -221,6 +221,10 @@ public function addConstraint($constraint_name, $options = NULL);
   /**
    * Determines whether the data value is internal.
    *
+   * Internal properties will not be returned during the normalization procces.
+   * By default, calculated properties will be internal unless otherwise
+   * specified.
+   *
    * @return bool
    *   Whether the data value is internal.
    */