diff -u b/core/lib/Drupal/Core/Entity/EntityType.php b/core/lib/Drupal/Core/Entity/EntityType.php
--- b/core/lib/Drupal/Core/Entity/EntityType.php
+++ b/core/lib/Drupal/Core/Entity/EntityType.php
@@ -19,11 +19,4 @@
 
   /**
-   * Indicates whether the entity definition should be internal.
-   *
-   * @var bool
-   */
-  protected $internal = FALSE;
-
-  /**
    * Indicates whether entities should be statically cached.
    *
@@ -163,4 +156,11 @@
 
   /**
+   * Indicates whether the entity data is internal.
+   *
+   * @var bool
+   */
+  protected $internal = FALSE;
+
+  /**
    * Indicates whether entities of this type have multilingual support.
    *
diff -u b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
--- b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
@@ -118,14 +118,4 @@
 
   /**
-   * Indicates whether the entity definition should be internal.
-   *
-   * @return bool
-   *   TRUE if the entity definition should be internal.
-   *
-   * @see \Drupal\Core\TypedData\DataDefinitionInterface::isInternal()
-   */
-  public function isInternal();
-
-  /**
    * Indicates whether entities should be statically cached.
    *
@@ -574,4 +564,22 @@
 
   /**
+   * Indicates whether the entity data is internal.
+   *
+   * This can be used in a scenario when it is not desirable to expose data of
+   * this entity type to an external system.
+   *
+   * The implications of this method are left to the discretion of the caller.
+   * For example, a module providing an HTTP API may not expose entities of
+   * this type or a custom entity reference field settings form may deprioritize
+   * entities of this type in a select list.
+   *
+   * @return bool
+   *   TRUE if the entity data is internal. FALSE otherwise.
+   *
+   * @see \Drupal\Core\TypedData\DataDefinitionInterface::isInternal()
+   */
+  public function isInternal();
+
+  /**
    * Indicates whether entities of this type have multilingual support.
    *
diff -u b/core/tests/Drupal/KernelTests/Core/Entity/EntityTypedDataDefinitionTest.php b/core/tests/Drupal/KernelTests/Core/Entity/EntityTypedDataDefinitionTest.php
--- b/core/tests/Drupal/KernelTests/Core/Entity/EntityTypedDataDefinitionTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Entity/EntityTypedDataDefinitionTest.php
@@ -162,7 +162,7 @@
   }
 
   /**
-   * Provides cases for testEntityDefinitionIsInternal.
+   * Provides test cases for testEntityDefinitionIsInternal.
    */
   public function entityDefinitionIsInternalProvider() {
     return [