diff -u b/core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginConfigurationTest.php b/core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginConfigurationTest.php
--- b/core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginConfigurationTest.php
+++ b/core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginConfigurationTest.php
@@ -41,8 +41,9 @@
    */
   public function mergeProvider() {
     return [
-      // Add new configuration.
+      // Tests adding new configuration to a migration.
       [
+        // New configuration.
         [
           'source' => [
             'constants' => [
@@ -50,6 +51,7 @@
             ],
           ],
         ],
+        // Expected final source configuration.
         [
           'plugin' => 'd7_blocked_ips',
           'constants' => [
@@ -57,13 +59,15 @@
           ],
         ],
       ],
-      // Override existing configuration.
+      // Tests overriding pre-existing configuration in a migration.
       [
+        // New configuration.
         [
           'source' => [
             'plugin' => 'a_different_plugin',
           ],
         ],
+        // Expected final source configuration.
         [
           'plugin' => 'a_different_plugin',
         ],