diff -u b/core/modules/migrate/src/Audit/AuditException.php b/core/modules/migrate/src/Audit/AuditException.php
--- b/core/modules/migrate/src/Audit/AuditException.php
+++ b/core/modules/migrate/src/Audit/AuditException.php
@@ -4,7 +4,10 @@
 
 use Drupal\migrate\Plugin\MigrationInterface;
 
-class AuditException extends \Exception {
+/**
+ * Exception thrown when a migration audit fails.
+ */
+class AuditException extends \RuntimeException {
 
   /**
    * AuditException constructor.
@@ -12,7 +15,7 @@
    * @param \Drupal\migrate\Plugin\MigrationInterface $migration
    *   The migration that caused the exception.
    * @param string $message
-   *   The reason for the failure.
+   *   The reason the audit failed.
    * @param \Exception $previous
    *   (optional) The previous exception.
    */
diff -u b/core/modules/migrate/src/Audit/AuditorInterface.php b/core/modules/migrate/src/Audit/AuditorInterface.php
--- b/core/modules/migrate/src/Audit/AuditorInterface.php
+++ b/core/modules/migrate/src/Audit/AuditorInterface.php
@@ -4,6 +4,14 @@
 
 use Drupal\migrate\Plugin\MigrationInterface;
 
+/**
+ * Defines an interface for migration auditors.
+ *
+ * A migration auditor is a class which can examine a migration to determine if
+ * it will cause conflicts with data already existing in the destination system.
+ * What kind of auditing it does, and how it does it, is up to the implementing
+ * class.
+ */
 interface AuditorInterface {
 
   /**
@@ -12,7 +20,12 @@
    * @param \Drupal\migrate\Plugin\MigrationInterface $migration
    *   The migration to audit.
    *
+   * @throws \Drupal\migrate\Audit\AuditException if the audit fails.
+   *
    * @return array
+   *   If the audit detects no potential conflicts, an empty array is returned.
+   *   Otherwise, the return value will be an array with a single element: a
+   *   string explaining the nature of the conflict, keyed by the migration ID.
    */
   public function audit(MigrationInterface $migration);
 
@@ -23,6 +36,8 @@
    *   The migrations to audit.
    *
    * @return array
+   *   An array of potential conflict information, keyed by migration ID. See
+   *   audit() for more information.
    */
   public function auditMultiple(array $migrations);
 
diff -u b/core/modules/migrate/src/Audit/IdAuditor.php b/core/modules/migrate/src/Audit/IdAuditor.php
--- b/core/modules/migrate/src/Audit/IdAuditor.php
+++ b/core/modules/migrate/src/Audit/IdAuditor.php
@@ -2,10 +2,12 @@
 
 namespace Drupal\migrate\Audit;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Drupal\migrate\Plugin\MigrationInterface;
 
+/**
+ * Audits migrations that create content entities in the destination system.
+ */
 class IdAuditor implements AuditorInterface {
 
   /**
@@ -47,6 +49,16 @@
     return $conflicts;
   }
 
+  /**
+   * Builds a conflict array.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The migration that has the conflict.
+   *
+   * @return array
+   *   The conflict. See \Drupal\migrate\Audit\AuditorInterface::audit() for
+   *   information on the format of the return value.
+   */
   public static function conflict(MigrationInterface $migration) {
     $base_id = $migration->getBaseId();
 
diff -u b/core/modules/migrate/src/Audit/MaximumValueInterface.php b/core/modules/migrate/src/Audit/MaximumValueInterface.php
--- b/core/modules/migrate/src/Audit/MaximumValueInterface.php
+++ b/core/modules/migrate/src/Audit/MaximumValueInterface.php
@@ -2,8 +2,25 @@
 
 namespace Drupal\migrate\Audit;
 
+/**
+ * Defines an interface for destination and ID maps which track a highest ID.
+ *
+ * When implemented by destination plugins, max() should return the highest ID
+ * of the destination entity type that exists in the system. So, for example,
+ * the entity:node plugin should return the highest node ID that exists,
+ * regardless of whether it was created by a migration.
+ *
+ * When implemented by an ID map, max() should return the highest migrated ID
+ * of the destination entity type.
+ */
 interface MaximumValueInterface {
 
+  /**
+   * Returns the highest ID value tracked by the implementing plugin.
+   *
+   * @return int
+   *   The highest ID.
+   */
   public function max();
 
 }