diff --git a/core/lib/Drupal/Core/Field/BundleFieldDefinition.php b/core/lib/Drupal/Core/Field/BundleFieldDefinition.php
index 86390fa606..a3c6a85e13 100644
--- a/core/lib/Drupal/Core/Field/BundleFieldDefinition.php
+++ b/core/lib/Drupal/Core/Field/BundleFieldDefinition.php
@@ -3,7 +3,22 @@
 namespace Drupal\Core\Field;
 
 /**
- * A class for defining bundle fields.
+ * A class for defining entity bundle fields.
+ *
+ * This class defines a builder for both a field definition and a field storage
+ * definition for a bundle field. A bundle field is different from a base field
+ * in that it may exist only for one or more bundles of an entity type.
+ *
+ * Bundle fields can defined in code using hook_entity_bundle_field_info or by
+ * implementing FieldableEntityInterface::bundleFieldDefinitions. Once a bundle
+ * field definition has been defined, if it is not computed and requires storage
+ * hook_entity_field_storage_info must also be implemented.
+ *
+ * @see \Drupal\Core\Entity\FieldableEntityInterface::bundleFieldDefinitions
+ * @see \Drupal\Core\Field\FieldDefinitionInterface
+ * @see \Drupal\Core\Field\FieldStorageDefinitionInterface
+ * @see hook_entity_bundle_field_info
+ * @see hook_entity_field_storage_info
  */
 class BundleFieldDefinition extends BaseFieldDefinition {
 
diff --git a/core/modules/system/src/Tests/Entity/EntityDefinitionTestTrait.php b/core/modules/system/src/Tests/Entity/EntityDefinitionTestTrait.php
index 380c1e66ed..6a4610da66 100644
--- a/core/modules/system/src/Tests/Entity/EntityDefinitionTestTrait.php
+++ b/core/modules/system/src/Tests/Entity/EntityDefinitionTestTrait.php
@@ -3,7 +3,7 @@
 namespace Drupal\system\Tests\Entity;
 
 use Drupal\Core\Field\BaseFieldDefinition;
-use Drupal\entity_test\FieldStorageDefinition;
+use Drupal\entity\BundleFieldDefinition;
 
 /**
  * Provides some test methods used to update existing entity definitions.
@@ -203,7 +203,7 @@ protected function removeBaseFieldIndex() {
    *   (optional) The field type for the new field. Defaults to 'string'.
    */
   protected function addBundleField($type = 'string') {
-    $definitions['new_bundle_field'] = FieldStorageDefinition::create($type)
+    $definitions['new_bundle_field'] = BundleFieldDefinition::create($type)
       ->setName('new_bundle_field')
       ->setLabel(t('A new bundle field'))
       ->setTargetEntityTypeId('entity_test_update');
diff --git a/core/modules/system/tests/modules/entity_schema_test/entity_schema_test.module b/core/modules/system/tests/modules/entity_schema_test/entity_schema_test.module
index 50bf3e3bb1..642fe1272e 100644
--- a/core/modules/system/tests/modules/entity_schema_test/entity_schema_test.module
+++ b/core/modules/system/tests/modules/entity_schema_test/entity_schema_test.module
@@ -7,7 +7,7 @@
 
 use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\Core\Field\BaseFieldDefinition;
-use Drupal\entity_test\FieldStorageDefinition;
+use Drupal\entity\BundleFieldDefinition;
 use Drupal\entity_test\Entity\EntityTestMulRev;
 
 /**
@@ -49,7 +49,7 @@ function entity_schema_test_entity_base_field_info(EntityTypeInterface $entity_t
  */
 function entity_schema_test_entity_field_storage_info(EntityTypeInterface $entity_type) {
   if ($entity_type->id() == 'entity_test') {
-    $definitions['custom_bundle_field'] = FieldStorageDefinition::create('string')
+    $definitions['custom_bundle_field'] = BundleFieldDefinition::create('string')
       ->setName('custom_bundle_field')
       ->setLabel(t('A custom bundle field'))
       ->setTargetEntityTypeId($entity_type->id());
diff --git a/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestBaseFieldDisplay.php b/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestBaseFieldDisplay.php
index 7fe9ce2112..de4532d94b 100644
--- a/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestBaseFieldDisplay.php
+++ b/core/modules/system/tests/modules/entity_test/src/Entity/EntityTestBaseFieldDisplay.php
@@ -4,7 +4,7 @@
 
 use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\Core\Field\BaseFieldDefinition;
-use Drupal\entity_test\FieldStorageDefinition;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
 
 /**
  * Defines a test entity class for base fields display.
@@ -77,7 +77,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
 
     $fields['test_display_multiple'] = BaseFieldDefinition::create('text')
       ->setLabel(t('A field with multiple values'))
-      ->setCardinality(FieldStorageDefinition::CARDINALITY_UNLIMITED)
+      ->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)
       ->setDisplayOptions('view', [
         'type' => 'text_default',
         'weight' => 12,