diff --git a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
index 375f8af26e..ca5a51326f 100644
--- a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
@@ -71,9 +71,9 @@ class MigrateUpgradeForm extends ConfirmFormBase {
   /**
    * List of extensions that do not need an upgrade path.
    *
-   * This property is an array where the keys are the version of Drupal from
-   * which we are upgrading and the values are arrays of extensions that do not
-   * need an upgrade path.
+   * This property is an array where the keys are the major Drupal core version
+   * from which we are upgrading, and the values are arrays of extension names
+   * that do not need an upgrade path.
    *
    * @var array[]
    */
@@ -759,6 +759,8 @@ public function buildConfirmForm(array $form, FormStateInterface $form_state) {
     // Get the source_module and destination_module from the field plugins.
     $definitions = $this->fieldPluginManager->getDefinitions();
     foreach ($definitions as $definition) {
+      // This is not strict so that we find field plugins with an annotation
+      // where the Drupal core version is an integer and when it is a string.
       if (in_array($version, $definition['core'])) {
         $source_module = $definition['source_module'];
         $destination_module = $definition['destination_module'];
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php
index 8ffe6a5d97..f8706ff6d3 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php
@@ -7,7 +7,7 @@
 use Drupal\Tests\migrate_drupal\Traits\CreateTestContentEntitiesTrait;
 
 /**
- * Provides a base class for testing migration upgrades in the UI.
+ * Provides a base class for testing a complete upgrade via the UI.
  */
 abstract class MigrateUpgradeExecuteTestBase extends MigrateUpgradeTestBase {
 
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeReviewPageTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeReviewPageTestBase.php
index 615c78c6fa..2c927dff18 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeReviewPageTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeReviewPageTestBase.php
@@ -4,9 +4,10 @@
 
 use Drupal\migrate_drupal\MigrationConfigurationTrait;
 use Drupal\Tests\migrate_drupal\Traits\CreateTestContentEntitiesTrait;
+use Drupal\Tests\WebAssert;
 
 /**
- * Provides a base class for testing the Upgrade review page.
+ * Provides a base class for testing the review step of the Upgrade form.
  */
 abstract class MigrateUpgradeReviewPageTestBase extends MigrateUpgradeTestBase {
 
@@ -14,9 +15,7 @@
   use CreateTestContentEntitiesTrait;
 
   /**
-   * Modules to enable.
-   *
-   * @var array
+   * {@inheritdoc}
    */
   public static $modules = [
     'language',
@@ -37,15 +36,17 @@
    *
    * The upgrade review form displays a list of modules that will be upgraded
    * and a list of modules that will not be upgraded. This test is to ensure
-   * that the review page works correctly for all Drupal 6 and Drupal 7 modules
-   * that have moved to core, ie Views, and for modules that were in Drupal 6 or
-   * Drupal 7 core but are not in Drupal 8 core, ie Overlay. To do this all
-   * modules in the source fixtures are enabled, except test and example
-   * modules. This means that we can test that the modules listed in the
-   * noUpgradePath is correct, because there will be no migrations with a
-   * source_module declared for that module. It is assumed that the modules in
-   * the test fixtures include all modules that have moved to or dropped from
-   * core.
+   * that the review page works correctly for all contributed Drupal 6 and
+   * Drupal 7 modules that have moved to core, e.g. Views, and for modules that
+   * were in Drupal 6 or Drupal 7 core but are not in Drupal 8 core, e.g.
+   * Overlay.
+   *
+   * To do this all modules in the source fixtures are enabled, except test and
+   * example modules. This means that we can test that the modules listed in the
+   * the $noUpgradePath property of the update form class are correct, since
+   * there will be no available migrations which declare those modules as their
+   * source_module. It is assumed that the test fixtures include all modules
+   * that have moved to or dropped from core.
    *
    * The upgrade review form will also display errors for each migration that
    * does not have a source_module definition. That function is not tested here.
@@ -79,7 +80,7 @@ public function testMigrateUpgradeReviewPage() {
     }
     $edits = $this->translatePostValues($edit);
 
-    // Enable all modules in the source except test and example modules but
+    // Enable all modules in the source except test and example modules, but
     // include simpletest.
     /** @var \Drupal\Core\Database\Query\SelectInterface $update */
     $update = $this->sourceDatabase->update('system')
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
index c17639cfc6..170a72505d 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -6,7 +6,7 @@
 use Drupal\migrate_drupal\MigrationConfigurationTrait;
 use Drupal\Tests\BrowserTestBase;
 use Drupal\Tests\migrate_drupal\Traits\CreateTestContentEntitiesTrait;
-
+use Drupal\Tests\WebAssert;
 /**
  * Provides a base class for testing migration upgrades in the UI.
  */
@@ -127,7 +127,7 @@ protected function translatePostValues(array $values) {
    * @param array $missing_paths
    *   An array of modules that will not be upgraded.
    */
-  protected function assertUpgradePaths($session, $available_paths, $missing_paths) {
+  protected function assertUpgradePaths(WebAssert $session, array $available_paths, array $missing_paths) {
     // Test the available migration paths.
     foreach ($available_paths as $available) {
       $session->elementExists('xpath', "//span[contains(@class, 'checked') and text() = '$available']");
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6ReviewPageTest.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6ReviewPageTest.php
index 71fd1cb748..ddcbc713e7 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6ReviewPageTest.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6ReviewPageTest.php
@@ -7,6 +7,7 @@
 /**
  * Tests migrate upgrade review page for Drupal 6.
  *
+ * @group migrate_drupal_6
  * @group migrate_drupal_ui
  */
 class MigrateUpgrade6ReviewPageTest extends MigrateUpgradeReviewPageTestBase {
@@ -66,7 +67,8 @@ protected function getAvailablePaths() {
       'upload',
       'user',
       'userreference',
-      // Modules that have no upgrade path.
+      // Include modules that do not have an upgrade path, defined in the
+      // $noUpgradePath property in MigrateUpgradeForm.
       'blog',
       'blogapi',
       'calendarsignup',
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
index dd94991291..d6ffe92355 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
@@ -133,7 +133,9 @@ protected function getAvailablePaths() {
       'upload',
       'user',
       'userreference',
-      // Modules that have no upgrade path.
+      // Include modules that do not have an upgrade path and are enabled in the
+      // source database, defined in the $noUpgradePath property
+      // in MigrateUpgradeForm.
       'date_api',
       'date_timezone',
       'i18n',
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7ReviewPageTest.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7ReviewPageTest.php
index 4cd8abbee3..2c2181aad2 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7ReviewPageTest.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7ReviewPageTest.php
@@ -7,6 +7,7 @@
 /**
  * Tests migrate upgrade review page for Drupal 7.
  *
+ * @group migrate_drupal_7
  * @group migrate_drupal_ui
  */
 class MigrateUpgrade7ReviewPageTest extends MigrateUpgradeReviewPageTestBase {
@@ -65,7 +66,8 @@ protected function getAvailablePaths() {
       'tracker',
       'update',
       'user',
-      // Modules that have no upgrade path.
+      // Include modules that do not have an upgrade path, defined in the
+      // $noUpgradePath property in MigrateUpgradeForm.
       'blog',
       'book',
       'bulk_export',
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
index 4450ee141b..9432f1db8f 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
@@ -136,7 +136,9 @@ protected function getAvailablePaths() {
       'taxonomy',
       'text',
       'user',
-      // Modules that have no upgrade path.
+      // Include modules that do not have an upgrade path and are enabled in the
+      // source database, defined in the $noUpgradePath property
+      // in MigrateUpgradeForm.
       'blog',
       'contextual',
       'date_api',