diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 8bb37c3f48..f09b187c76 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -191,6 +191,10 @@ function install_state_defaults() {
     // The last task that was completed during the previous installation
     // request.
     'completed_task' => NULL,
+    // Partial configuration cached during an installation from existing config.
+    'config' => NULL,
+    // The path to the configuration to install when installing from config.
+    'config_install' => NULL,
     // TRUE when there are valid config directories.
     'config_verified' => FALSE,
     // TRUE when there is a valid database connection.
@@ -797,7 +801,11 @@ function install_tasks($install_state) {
   ];
 
   if (!empty($install_state['config_install'])) {
-    // @todo add a load of commentary about what is happening.
+    // The chosen profile indicates that rather than installing a new site, an
+    // instance of the same site should be installed from the given
+    // configuration.
+    // That means we need to remove the steps installing the extensions and
+    // replace them with a configuration synchronisation step.
     unset($tasks['install_download_translation']);
     $key = array_search('install_profile_modules', array_keys($tasks), TRUE);
     unset($tasks['install_profile_modules']);