diff --git a/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php b/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
index 8d69418262..a23412f89a 100644
--- a/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php
@@ -335,7 +335,10 @@ public function rewind() {
    *
    * 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 system. A row will be processed if it has not already been imported,
+   * or the row needs an update, or it is above the highwater mark or the source
+   * row has changed. When set to be processed the row is also marked frozen
+   * and no further changes to the row source properties are allowed.
    *
    * 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) {