diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/field/field_type/TaxonomyTermReferenceItem.php b/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/field/field_type/TaxonomyTermReferenceItem.php
index 1ea9e2e..3997007 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/field/field_type/TaxonomyTermReferenceItem.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/field/field_type/TaxonomyTermReferenceItem.php
@@ -36,13 +36,16 @@
  *   default_formatter = "taxonomy_term_reference_link"
  * )
  */
-class TaxonomyTermReferenceItem extends EntityReferenceItem implements ConfigFieldItemInterface, AllowedValuesInterface{
+class TaxonomyTermReferenceItem extends EntityReferenceItem implements ConfigFieldItemInterface, AllowedValuesInterface {
 
   /**
    * {@inheritdoc}
    */
   public function getPossibleValues(AccountInterface $account = NULL) {
-    return array_keys($this->getPossibleOptions($account));
+    // Flatten options firstly, because Possible Options may contain group
+    // arrays.
+    $flatten_options = $this->flattenOptions($this->getPossibleOptions($account));
+    return array_keys($flatten_options);
   }
 
   /**
@@ -56,7 +59,10 @@ public function getPossibleOptions(AccountInterface $account = NULL) {
    * {@inheritdoc}
    */
   public function getSettableValues(AccountInterface $account = NULL) {
-    return array_keys($this->getSettableOptions($account));
+   // Flatten options firstly, because Settable Options may contain group
+   // arrays.
+    $flatten_options = $this->flattenOptions($this->getSettableOptions($account));
+    return array_keys($flatten_options);
   }
 
   /**
@@ -113,11 +119,11 @@ public function settingsForm(array $form, array &$form_state, $has_data) {
     }
 
     $settings = $this->getFieldSettings();
-    $form = array();
-    $form['#tree'] = TRUE;
+    $element = array();
+    $element['#tree'] = TRUE;
 
     foreach ($settings['allowed_values'] as $delta => $tree) {
-      $form['allowed_values'][$delta]['vocabulary'] = array(
+      $element['allowed_values'][$delta]['vocabulary'] = array(
         '#type' => 'select',
         '#title' => t('Vocabulary'),
         '#default_value' => $tree['vocabulary'],
@@ -126,13 +132,13 @@ public function settingsForm(array $form, array &$form_state, $has_data) {
         '#description' => t('The vocabulary which supplies the options for this field.'),
         '#disabled' => $has_data,
       );
-      $form['allowed_values'][$delta]['parent'] = array(
+      $element['allowed_values'][$delta]['parent'] = array(
         '#type' => 'value',
         '#value' => $tree['parent'],
       );
     }
 
-    return $form;
+    return $element;
   }
 
   /**
@@ -165,6 +171,11 @@ public function preSave() {
 
   /**
    * {@inheritdoc}
+   *
+   * Taxonomy field settings allow for either a single vocabulary ID, multiple
+   * vocabulary IDs, or sub-trees of a vocabulary to be specified as allowed
+   * values, although only the first of these is supported via the field UI.
+   * Confirm that terms entered as values meet at least one of these conditions.
    */
   public function getConstraints() {
     $constraint_manager = \Drupal::typedData()->getValidationConstraintManager();
@@ -184,4 +195,19 @@ public function getConstraints() {
     return $constraints;
   }
 
+  /**
+  * Flattens an array of allowed values.
+  *
+  * @param array $array
+  *   A single or multidimensional array.
+  *
+  * @return array
+  *   The flattened array.
+  */
+  protected function flattenOptions(array $array) {
+    $result = array();
+    array_walk_recursive($array, function($a, $b) use (&$result) { $result[$b] = $a; });
+    return $result;
+  }
+
 }
diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index 648befd..4ca5676 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -917,6 +917,24 @@ function taxonomy_autocomplete_validate($element, &$form_state) {
 }
 
 /**
+ * @defgroup taxonomy_index Taxonomy indexing
+ * @{
+ * Functions to maintain taxonomy indexing.
+ *
+ * Taxonomy uses default field storage to store canonical relationships
+ * between terms and fieldable entities. However its most common use case
+ * requires listing all content associated with a term or group of terms
+ * sorted by creation date. To avoid slow queries due to joining across
+ * multiple node and field tables with various conditions and order by criteria,
+ * we maintain a denormalized table with all relationships between terms,
+ * published nodes and common sort criteria such as sticky and created.
+ * This is used as a lookup table by taxonomy_select_nodes(). When using other
+ * field storage engines or alternative methods of denormalizing this data
+ * you should set the taxonomy.settings:maintain_index_table to '0' to avoid
+ * unnecessary writes in SQL.
+ */
+
+/**
  * Implements hook_node_insert().
  */
 function taxonomy_node_insert(EntityInterface $node) {