diff --git a/modules/system/system.install b/modules/system/system.install
index b162847..bc7b95b 100644
--- a/modules/system/system.install
+++ b/modules/system/system.install
@@ -2807,6 +2807,10 @@ function system_update_7061(&$sandbox) {
     // first of these will be moved to the new {file_managed} table (and all
     // references will be updated to point to it), since duplicate file URIs
     // are not allowed in Drupal 7.
+    // Since the Drupal 6 to 7 upgrade path leaves the {files} table behind
+    // after it's done, custom or contributed modules which need to migrate
+    // file references of their own can use a similar query to determine the
+    // file IDs that duplicate filepaths were mapped to.
     $sandbox['duplicate_filepath_fids_to_use'] = db_query("SELECT filepath, MIN(fid) FROM {files} GROUP BY filepath HAVING COUNT(*) > 1")->fetchAllKeyed();
 
     // Initialize batch update information.