diff --git a/lib/filelib.php b/lib/filelib.php
index d3eeb27ab70..933a1cab8ea 100644
--- a/lib/filelib.php
+++ b/lib/filelib.php
@@ -713,6 +713,9 @@ function file_get_submitted_draft_itemid($elname) {
 /**
  * Restore the original source field from draft files
  *
+ * Do not use this function because it makes field files.source inconsistent
+ * for draft area files. This function will be deprecated in 2.6
+ *
  * @param stored_file $storedfile This only works with draft files
  * @return stored_file
  */
@@ -860,6 +863,8 @@ function file_save_draft_area_files($draftitemid, $contextid, $component, $filea
             }
 
             // Updated file source
+            // Field files.source for draftarea files contains serialised object with source and original information.
+            // We only store the source part of it for non-draft file area.
             $newsource = $newfile->get_source();
             if ($source = @unserialize($newfile->get_source())) {
                 $newsource = $source->source;
@@ -898,6 +903,7 @@ function file_save_draft_area_files($draftitemid, $contextid, $component, $filea
             $file_record = array('contextid'=>$contextid, 'component'=>$component, 'filearea'=>$filearea, 'itemid'=>$itemid, 'timemodified'=>time());
             if ($source = @unserialize($file->get_source())) {
                 // Field files.source for draftarea files contains serialised object with source and original information.
+                // We only store the source part of it for non-draft file area.
                 $file_record['source'] = $source->source;
             }
 
diff --git a/lib/filestorage/stored_file.php b/lib/filestorage/stored_file.php
index d78aac18c85..3a27e81cf70 100644
--- a/lib/filestorage/stored_file.php
+++ b/lib/filestorage/stored_file.php
@@ -214,7 +214,10 @@ class stored_file {
      * Replaces the fields that might have changed when file was overriden in filepicker:
      * reference, contenthash, filesize
      *
-     * Note that field source must be updated separately
+     * Note that field 'source' must be updated separately because
+     * it has different format for draft and non-draft areas and
+     * this function will usually be used to replace non-draft area
+     * file with draft area file.
      *
      * @param stored_file $newfile
      * @throws coding_exception