diff --git a/core/modules/migrate/src/Event/MigrateMemoryLimitEvent.php b/core/modules/migrate/src/Event/MigrateMemoryLimitEvent.php
index 85b404daa0..81aa17c11e 100644
--- a/core/modules/migrate/src/Event/MigrateMemoryLimitEvent.php
+++ b/core/modules/migrate/src/Event/MigrateMemoryLimitEvent.php
@@ -33,6 +33,8 @@ class MigrateMemoryLimitEvent extends Event {
   /**
    * The phase of memory reclamation.
    *
+   * The phases are defined in \Drupal\migrate\MemoryManagerInterface.
+   *
    * @var string
    */
   protected $phase;