diff --git a/core/modules/layout/lib/Drupal/layout/Config/BoundDisplayInterface.php b/core/modules/layout/lib/Drupal/layout/Config/BoundDisplayInterface.php
index 1af540c..beceff2 100644
--- a/core/modules/layout/lib/Drupal/layout/Config/BoundDisplayInterface.php
+++ b/core/modules/layout/lib/Drupal/layout/Config/BoundDisplayInterface.php
@@ -8,6 +8,15 @@
 
 use Drupal\layout\Plugin\LayoutInterface;
 
+/**
+ * Interface for a Display object that is coupled to a specific layout plugin.
+ *
+ * This display contains references both to block instances and a specific
+ * layout plugin. The blocks are assigned to the regions declared by that
+ * layout. Bound Displays are used to serve real pages at request time.
+ *
+ * @see \Drupal\layout\Config\DisplayInterface
+ */
 interface BoundDisplayInterface extends DisplayInterface {
 
   /**
@@ -52,10 +61,14 @@ public function getAllSortedBlocks();
   public function getLayoutInstance();
 
   /**
-   * Adjusts this display's block information in response to a changed layout.
+   * Adjusts this display's block placement to work with the provided layout.
+   *
+   * Essentially a shortcut that calls DisplayInterface::mapBlocksToLayout(),
+   * saves the result in the appropriate object property, and finally calls
+   * BoundDisplayInterface::setLayout().
    *
    * @param \Drupal\layout\Plugin\LayoutInterface $layout
-   *   The new layout.
+   *   The new layout to which blocks should be remapped.
    *
    * @see \Drupal\layout\Config\DisplayInterface::mapBlocksToLayout()
    */
diff --git a/core/modules/layout/lib/Drupal/layout/Config/DisplayBase.php b/core/modules/layout/lib/Drupal/layout/Config/DisplayBase.php
index 95c07bb..ec11520 100644
--- a/core/modules/layout/lib/Drupal/layout/Config/DisplayBase.php
+++ b/core/modules/layout/lib/Drupal/layout/Config/DisplayBase.php
@@ -127,4 +127,17 @@ public function mapBlocksToLayout(LayoutInterface $layout) {
 
     return $remapped_config;
   }
+
+  /**
+   * Implements DisplayInterface::getAllRegionTypes().
+   *
+   * @return array
+   */
+  public function getAllRegionTypes() {
+    $types = array();
+    foreach ($this->blockInfo as $info) {
+      $types[$info['region-type']] = TRUE;
+    }
+    return array_keys($types);
+  }
 }
diff --git a/core/modules/layout/lib/Drupal/layout/Config/DisplayInterface.php b/core/modules/layout/lib/Drupal/layout/Config/DisplayInterface.php
index 1337077..06e1635 100644
--- a/core/modules/layout/lib/Drupal/layout/Config/DisplayInterface.php
+++ b/core/modules/layout/lib/Drupal/layout/Config/DisplayInterface.php
@@ -8,6 +8,19 @@
 
 use Drupal\layout\Plugin\LayoutInterface;
 
+/**
+ * Interface describing a Display configuration object.
+ *
+ * Displays are configuration that describe the placement of block instances
+ * in regions, as well as certain ways (for example, caching and visibility) of
+ * those block instances within those regions.
+ *
+ * Drupal ships with two major families of Display objects:
+ *
+ * @see \Drupal\layout\Config\BoundDisplayInterface
+ * @see \Drupal\layout\Config\UnboundDisplayInterface
+ *
+ */
 interface DisplayInterface {
 
   /**
@@ -36,4 +49,13 @@ public function getAllBlockInfo();
    *   is returned by DisplayInterface::getAllBlockInfo().
    */
   public function mapBlocksToLayout(LayoutInterface $layout);
+
+  /**
+   * Returns the names of all region types to which blocks are assigned.
+   *
+   * @return array
+   *   An indexed array of unique region type names, or an empty array if no
+   *   region types were assigned.
+   */
+  public function getAllRegionTypes();
 }
diff --git a/core/modules/layout/lib/Drupal/layout/Config/UnboundDisplayInterface.php b/core/modules/layout/lib/Drupal/layout/Config/UnboundDisplayInterface.php
index d709db1..7139397 100644
--- a/core/modules/layout/lib/Drupal/layout/Config/UnboundDisplayInterface.php
+++ b/core/modules/layout/lib/Drupal/layout/Config/UnboundDisplayInterface.php
@@ -8,6 +8,18 @@
 
 use Drupal\layout\Plugin\LayoutInterface;
 
+/**
+ * Interface for a Display that is not coupled with any layout plugin.
+ *
+ * Unbound Displays contain references to blocks, but not to any particular
+ * layout plugin. Their primary use case is to express a set of relative block
+ * placements without necessitating any particular layout be present. This
+ * allows upstream (module and distribution) developers to express a visual
+ * composition of blocks without knowing anything about the layout plugins a
+ * particular site has available.
+ *
+ * @see \Drupal\layout\Config\DisplayInterface
+ */
 interface UnboundDisplayInterface extends DisplayInterface {
   /**
    * Bind this UnboundDisplay to a particular display.
diff --git a/core/modules/layout/lib/Drupal/layout/Plugin/Core/Entity/Display.php b/core/modules/layout/lib/Drupal/layout/Plugin/Core/Entity/Display.php
index 3b5a004..7a38ed2 100644
--- a/core/modules/layout/lib/Drupal/layout/Plugin/Core/Entity/Display.php
+++ b/core/modules/layout/lib/Drupal/layout/Plugin/Core/Entity/Display.php
@@ -130,10 +130,7 @@ protected function sortBlocks() {
   }
 
   /**
-   * Perform block remapping per mapBlocksToLayout(), but mutate this object
-   * with the remapping results instead of returning them.
-   *
-   * @see \Drupal\layout\Config\DisplayBase::mapBlocksToLayout()
+   * Implements BoundDisplayInterface::remapToLayout().
    *
    * @param \Drupal\layout\Plugin\LayoutInterface $layout
    */
diff --git a/core/modules/layout/lib/Drupal/layout/Tests/DisplayInternalLogicTest.php b/core/modules/layout/lib/Drupal/layout/Tests/DisplayInternalLogicTest.php
index 47ca634..07a77bf 100644
--- a/core/modules/layout/lib/Drupal/layout/Tests/DisplayInternalLogicTest.php
+++ b/core/modules/layout/lib/Drupal/layout/Tests/DisplayInternalLogicTest.php
@@ -13,7 +13,6 @@
 
 /**
  * Tests the API and internal logic offered by Displays.
- *
  */
 class DisplayInternalLogicTest extends WebTestBase {