diff --git a/core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php b/core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
index 91fdce4..99be1f0 100644
--- a/core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
+++ b/core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
@@ -51,8 +51,8 @@ public function getConfiguration() {
    * {@inheritdoc}
    */
   public function setConfiguration(array $configuration) {
-    // Resolve BC level configurations, if any.
-    $this->resolveBcConfiguration($configuration);
+    // Resolve backward compatibility level configurations, if any.
+    $this->resolveBackwardCompatibilityConfiguration($configuration);
 
     // Merge in defaults.
     $this->configuration = NestedArray::mergeDeep(
@@ -60,8 +60,8 @@ public function setConfiguration(array $configuration) {
       $configuration
     );
 
-    // Assure a BC level configuration.
-    $this->assureBcConfiguration();
+    // Ensure a backward compatibility level configuration.
+    $this->ensureBackwardCompatibilityConfiguration();
   }
 
   /**
@@ -94,7 +94,7 @@ public function submitConfigurationForm(array &$form, FormStateInterface $form_s
   public function entityQueryAlter(SelectInterface $query) { }
 
   /**
-   * Moves the BC level configurations in the right place.
+   * Moves the backward compatibility level configurations in the right place.
    *
    * In order to keep backward compatibility, we copy all settings, except
    * 'target_type', 'handler' and 'entity' under 'handler_settings', following
@@ -120,7 +120,7 @@ public function entityQueryAlter(SelectInterface $query) { }
    *
    * @deprecated Scheduled for removal in Drupal 9.0.x.
    */
-  protected function resolveBcConfiguration(array &$configuration) {
+  protected function resolveBackwardCompatibilityConfiguration(array &$configuration) {
     if (isset($this->defaultConfiguration()['handler_settings'])) {
       throw new \InvalidArgumentException("{$this->getPluginDefinition()['class']}::defaultConfiguration() should not contain a 'handler_settings' key. All settings should be placed in the root level.");
     }
@@ -130,7 +130,7 @@ protected function resolveBcConfiguration(array &$configuration) {
       if (!is_array($configuration['handler_settings'])) {
         throw new \InvalidArgumentException("The setting 'handler_settings' is reserved and cannot be used.");
       }
-      @trigger_error("Settings under 'handler_settings' are deprecated since version 8.3.x and will be removed before 9.0.0. Move the settings in the root of the configuration array.", E_USER_DEPRECATED);
+      @trigger_error("Providing settings under 'handler_settings' is deprecated since version 8.3.x and will be removed before 9.0.0. Move the settings in the root of the configuration array.", E_USER_DEPRECATED);
 
       // Settings passed in the root level take precedence over BC settings.
       $configuration += $configuration['handler_settings'];
@@ -139,11 +139,11 @@ protected function resolveBcConfiguration(array &$configuration) {
   }
 
   /**
-   * Assures a BC level configuration.
+   * Ensures a backward compatibility level configuration.
    *
    * @deprecated Scheduled for removal in Drupal 9.0.x.
    */
-  protected function assureBcConfiguration() {
+  protected function ensureBackwardCompatibilityConfiguration() {
     // Synchronize back 'handler_settings'.
     foreach ($this->configuration as $key => $value) {
       // Filter out keys that belong strictly to the root level.
diff --git a/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php b/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php
index 61372d1..65556c3 100644
--- a/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php
@@ -35,7 +35,7 @@ protected function setUp() {
    * Tests invalid default configuration.
    *
    * @covers ::defaultConfiguration
-   * @covers ::resolveBcConfiguration
+   * @covers ::resolveBackwardCompatibilityConfiguration
    * @expectedException \InvalidArgumentException
    * @expectedExceptionMessage TestSelectionWithInvalidDefaultConfiguration::defaultConfiguration() should not contain a 'handler_settings' key. All settings should be placed in the root level.
    */
@@ -51,7 +51,7 @@ public function testInvalidDefaultConfiguration() {
    * Tests the selection handler with malformed 'handler_settings' value.
    *
    * @covers ::setConfiguration
-   * @covers ::resolveBcConfiguration
+   * @covers ::resolveBackwardCompatibilityConfiguration
    * @expectedException \InvalidArgumentException
    * @expectedExceptionMessage The setting 'handler_settings' is reserved and cannot be used.
    */
@@ -125,6 +125,8 @@ public function providerTestSetConfiguration() {
    *
    * @dataProvider providerTestSetConfiguration
    * @covers ::setConfiguration
+   * @covers ::resolveBackwardCompatibilityConfiguration
+   * @covers ::ensureBackwardCompatibilityConfiguration
    *
    * @param array $options
    *   The configuration passed to the plugin.
@@ -161,7 +163,8 @@ public function testSetConfiguration($options) {
    * Tests the selection handler plugin BC structure.
    *
    * @covers ::setConfiguration
-   * @covers ::resolveBcConfiguration
+   * @covers ::resolveBackwardCompatibilityConfiguration
+   * @covers ::ensureBackwardCompatibilityConfiguration
    */
   public function testSetConfigurationBcLevel() {
     $config = [
@@ -201,20 +204,20 @@ public function testSetConfigurationBcLevel() {
    * Tests deprecation error triggering.
    *
    * @covers ::setConfiguration
-   * @covers ::resolveBcConfiguration
+   * @covers ::resolveBackwardCompatibilityConfiguration
    */
   public function testDeprecationErrorTriggering() {
     // Configuration with no BC level.
     $config = ['setting1' => TRUE];
     new TestSelection($config, 'test_selector', []);
     // Check that deprecation error has not been triggered.
-    $this->assertNoError("Settings under 'handler_settings' are deprecated since version 8.3.x and will be removed before 9.0.0. Move the settings in the root of the configuration array.", E_USER_DEPRECATED);
+    $this->assertNoError("Providing settings under 'handler_settings' is deprecated since version 8.3.x and will be removed before 9.0.0. Move the settings in the root of the configuration array.", E_USER_DEPRECATED);
 
     // Configuration with BC level.
     $config = ['handler_settings' => ['setting1' => TRUE]];
     new TestSelection($config, 'test_selector', []);
     // Check that deprecation error has been triggered.
-    $this->assertError("Settings under 'handler_settings' are deprecated since version 8.3.x and will be removed before 9.0.0. Move the settings in the root of the configuration array.", E_USER_DEPRECATED);
+    $this->assertError("Providing settings under 'handler_settings' is deprecated since version 8.3.x and will be removed before 9.0.0. Move the settings in the root of the configuration array.", E_USER_DEPRECATED);
   }
 
   /**
@@ -228,7 +231,7 @@ public function testDeprecationErrorTriggering() {
   protected function assertError($message, $severity) {
     $assertion_message = "Error '$message' (severity $severity) was triggered.";
     foreach ($this->errors as $error) {
-      if ($error['message'] === $message && $error['severity'] == $severity) {
+      if ($error['message'] === $message && $error['severity'] === $severity) {
         $this->assertTrue(TRUE, $assertion_message);
         return;
       }
@@ -247,7 +250,7 @@ protected function assertError($message, $severity) {
   protected function assertNoError($message, $severity) {
     $assertion_message = "Error '$message' (severity $severity) was not triggered.";
     foreach ($this->errors as $error) {
-      if ($error['message'] === $message && $error['severity'] == $severity) {
+      if ($error['message'] === $message && $error['severity'] === $severity) {
         $this->fail($assertion_message);
         return;
       }