diff --git a/core/lib/Drupal/Core/Extension/module.api.php b/core/lib/Drupal/Core/Extension/module.api.php
index 9abea30b9fc8a5bbadfed0fd7db0b1d568c1c490..7f65fea19df9d4e831c8d503c660970071e055f4 100644
--- a/core/lib/Drupal/Core/Extension/module.api.php
+++ b/core/lib/Drupal/Core/Extension/module.api.php
@@ -894,6 +894,9 @@ function hook_updater_info_alter(&$updaters) {
  * Module dependencies do not belong to these installation requirements,
  * but should be defined in the module's .info.yml file.
  *
+ * During installation (when $phase == 'install'), if you need to load a class
+ * from your module, you'll need to include the class file directly.
+ *
  * The 'runtime' phase is not limited to pure installation requirements
  * but can also be used for more general status information like maintenance
  * tasks and security issues.