diff --git a/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php b/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
index 8d69418262..bb4c9bfadf 100644
--- a/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
@@ -333,9 +333,20 @@ public function rewind() {
   /**
    * {@inheritdoc}
    *
-   * The migration iterates over rows returned by the source plugin. This
-   * method determines the next row which will be processed and imported into
-   * the system.
+   * The migration iterates over rows returned by the source plugin. For each
+   * row, the map row, if it exists is deleted and then the prepareRow method
+   * and hook_prepare_row are executed. The next step is to determine if the row
+   * will be processed.
+   * A row will be processed in any of these cases:
+   * - the row has not already been imported
+   * - the row needs an update
+   * - the row is above the highwater mark
+   * - the source row has changed
+   * A row is considered changed only if track_changes is set on the source
+   * plugin and the source values for the row have changed since the last
+   * import. When set to be processed, the row is also marked frozen and no
+   * further changes to the row source properties are allowed. The last
+   * step is to set the highwater value, if highwater is in use.
    *
    * The method tracks the source and destination IDs using the ID map plugin.
    *
diff --git a/core/modules/migrate/src/Row.php b/core/modules/migrate/src/Row.php
index 08f41ba33e..6b6f1ad9d0 100644
--- a/core/modules/migrate/src/Row.php
+++ b/core/modules/migrate/src/Row.php
@@ -167,7 +167,7 @@ public function getSource() {
   }
 
   /**
-   * Sets a source property.
+   * Sets a source property if the row is not frozen.
    *
    * This can only be called from the source plugin.
    *
@@ -177,6 +177,8 @@ public function getSource() {
    *   The property value to set on the source.
    *
    * @throws \Exception
+   *
+   * @see \Drupal\migrate\Plugin\migrate\source\SourcePluginBase::next
    */
   public function setSourceProperty($property, $data) {
     if ($this->frozen) {