diff --git a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
index 14a0a7a..6aaf8ae 100644
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -174,7 +174,11 @@ public static function create(ContainerInterface $container, array $configuratio
   /**
    * Retrieves the hash of the source identifier values.
    *
-   * It is public only for testing purposes.
+   * It is public only for testing purposes. The source id values should be
+   * ordered in the same order as the data returned by a source plugin's
+   * getIds() method.
+   *
+   * @internal
    *
    * @param array $source_id_values
    *   The source identifiers