diff --git a/core/modules/forum/tests/src/Kernel/Migrate/d6/MigrateForumTest.php b/core/modules/forum/tests/src/Kernel/Migrate/d6/MigrateForumTest.php
index 0d3c447..454f58c 100644
--- a/core/modules/forum/tests/src/Kernel/Migrate/d6/MigrateForumTest.php
+++ b/core/modules/forum/tests/src/Kernel/Migrate/d6/MigrateForumTest.php
@@ -50,7 +50,7 @@ protected function setUp() {
    */
   public function testForumMigration() {
     $node = Node::load(19);
-    $this->assertEquals([0 => ['target_id' => '7']], $node->get('taxonomy_forums')->getValue());
+    $this->assertEquals(8, $node->taxonomy_forums->target_id);
   }
 
 }
diff --git a/core/modules/migrate_drupal/migrate_drupal.module b/core/modules/migrate_drupal/migrate_drupal.module
index 22ed76b..10aa72b 100644
--- a/core/modules/migrate_drupal/migrate_drupal.module
+++ b/core/modules/migrate_drupal/migrate_drupal.module
@@ -57,6 +57,12 @@ function migrate_drupal_migration_plugins_alter(&$definitions) {
         $plugin_ids = ['d6_term_node:' . $source_vid, 'd6_term_node_revision:' . $source_vid];
         foreach ($plugin_ids as $plugin_id) {
           if (isset($definitions[$plugin_id])) {
+            // The Forum module is expecting 'taxonomy_forums' as the field name
+            // for the forum nodes. The 'forum_vocabulary' source property is
+            // evaluated in Drupal\taxonomy\Plugin\migrate\source\d6\Vocabulary
+            // and is set to true if the vocabulary vid being migrated is the
+            // same as the one in the 'forum_nav_vocabulary' variable on the
+            // source site.
             $destination_vid = $row->getSourceProperty('forum_vocabulary') ? 'taxonomy_forums' : $row->getDestinationProperty('vid');
             $definitions[$plugin_id]['process'][$destination_vid] = 'tid';
           }
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
index 8dea7d0..1bc5aa8 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
@@ -44,7 +44,7 @@ protected function getEntityCounts() {
       'contact_form' => 5,
       'configurable_language' => 5,
       'editor' => 2,
-      'field_config' => 73,
+      'field_config' => 72,
       'field_storage_config' => 48,
       'file' => 7,
       'filter_format' => 7,
@@ -59,7 +59,7 @@ protected function getEntityCounts() {
       'shortcut_set' => 1,
       'action' => 22,
       'menu' => 8,
-      'taxonomy_term' => 7,
+      'taxonomy_term' => 8,
       'taxonomy_vocabulary' => 6,
       'tour' => 4,
       'user' => 7,
diff --git a/core/modules/taxonomy/migration_templates/d6_vocabulary_entity_display.yml b/core/modules/taxonomy/migration_templates/d6_vocabulary_entity_display.yml
index 22e5004..53d7dc4 100644
--- a/core/modules/taxonomy/migration_templates/d6_vocabulary_entity_display.yml
+++ b/core/modules/taxonomy/migration_templates/d6_vocabulary_entity_display.yml
@@ -29,6 +29,9 @@ process:
       migration: d6_taxonomy_vocabulary
       source: vid
     -
+      # This plugin checks if the vocabulary being migrated is the one used for
+      # forums. If it is, the field name is set to 'taxonomy_forums' as it is
+      # what the Forum module is expecting. Otherwise, it is left unchanged.
       plugin: forum_vocabulary
 destination:
   plugin: component_entity_display
diff --git a/core/modules/taxonomy/migration_templates/d6_vocabulary_entity_form_display.yml b/core/modules/taxonomy/migration_templates/d6_vocabulary_entity_form_display.yml
index 8b70e4b..2cf20cd 100644
--- a/core/modules/taxonomy/migration_templates/d6_vocabulary_entity_form_display.yml
+++ b/core/modules/taxonomy/migration_templates/d6_vocabulary_entity_form_display.yml
@@ -33,6 +33,9 @@ process:
       migration: d6_taxonomy_vocabulary
       source: vid
     -
+      # This plugin checks if the vocabulary being migrated is the one used for
+      # forums. If it is, the field name is set to 'taxonomy_forums' as it is
+      # what the Forum module is expecting. Otherwise, it is left unchanged.
       plugin: forum_vocabulary
 destination:
   plugin: component_entity_form_display
diff --git a/core/modules/taxonomy/migration_templates/d6_vocabulary_field.yml b/core/modules/taxonomy/migration_templates/d6_vocabulary_field.yml
index 08b86f2..fb96fa3 100644
--- a/core/modules/taxonomy/migration_templates/d6_vocabulary_field.yml
+++ b/core/modules/taxonomy/migration_templates/d6_vocabulary_field.yml
@@ -17,6 +17,9 @@ process:
       migration: d6_taxonomy_vocabulary
       source: vid
     -
+      # This plugin checks if the vocabulary being migrated is the one used for
+      # forums. If it is, the field name is set to 'taxonomy_forums' as it is
+      # what the Forum module is expecting. Otherwise, it is left unchanged.
       plugin: forum_vocabulary
     -
       plugin: skip_on_empty
diff --git a/core/modules/taxonomy/migration_templates/d6_vocabulary_field_instance.yml b/core/modules/taxonomy/migration_templates/d6_vocabulary_field_instance.yml
index bb896c1..2e4f3a2 100644
--- a/core/modules/taxonomy/migration_templates/d6_vocabulary_field_instance.yml
+++ b/core/modules/taxonomy/migration_templates/d6_vocabulary_field_instance.yml
@@ -24,6 +24,9 @@ process:
       migration: d6_taxonomy_vocabulary
       source: vid
     -
+      # This plugin checks if the vocabulary being migrated is the one used for
+      # forums. If it is, the field name is set to 'taxonomy_forums' as it is
+      # what the Forum module is expecting. Otherwise, it is left unchanged.
       plugin: forum_vocabulary
     -
       plugin: skip_on_empty
diff --git a/core/modules/taxonomy/src/Plugin/migrate/process/ForumVocabulary.php b/core/modules/taxonomy/src/Plugin/migrate/process/ForumVocabulary.php
index db99652..be4b9cc 100644
--- a/core/modules/taxonomy/src/Plugin/migrate/process/ForumVocabulary.php
+++ b/core/modules/taxonomy/src/Plugin/migrate/process/ForumVocabulary.php
@@ -7,6 +7,14 @@
 use Drupal\migrate\Row;
 
 /**
+ * Checks if the vocabulary being migrated is the one used for forums.
+ *
+ * The forum module is expecting 'taxonomy_forums' as the field name for the
+ * forum nodes. The 'forum_vocabulary' source property is evaluated in
+ * Drupal\taxonomy\Plugin\migrate\source\d6\Vocabulary and is set to true if
+ * the vocabulary vid being migrated is the same as the one in the
+ * 'forum_nav_vocabulary' variable on the source site.
+ *
  * @MigrateProcessPlugin(
  *   id = "forum_vocabulary"
  * )
diff --git a/core/modules/taxonomy/src/Plugin/migrate/source/d6/Vocabulary.php b/core/modules/taxonomy/src/Plugin/migrate/source/d6/Vocabulary.php
index 0a64ef9..8924c33 100644
--- a/core/modules/taxonomy/src/Plugin/migrate/source/d6/Vocabulary.php
+++ b/core/modules/taxonomy/src/Plugin/migrate/source/d6/Vocabulary.php
@@ -69,9 +69,14 @@ public function prepareRow(Row $row) {
       ->fetchCol();
     $row->setSourceProperty('node_types', $node_types);
     $row->setSourceProperty('cardinality', ($row->getSourceProperty('tags') == 1 || $row->getSourceProperty('multiple') == 1) ? FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED : 1);
+
+    // If the vid of the vocabulary being migrated is equal to the vid in the
+    // 'forum_nav_vocabulary' variable, set the 'forum_vocabulary' source
+    // property to true so we can know this is the vocabulary used for forums.
     if ($this->variableGet('forum_nav_vocabulary', 0) == $row->getSourceProperty('vid')) {
       $row->setSourceProperty('forum_vocabulary', TRUE);
     }
+
     return parent::prepareRow($row);
   }