diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index cb4ad75b21..52e9ff9078 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -894,6 +894,7 @@ typedef struct AVPanScan{
 \
     /**\
      * motion referece frame index\
+     * the order in which these are stored can depends on the codec.\
      * - encoding: Set by user.\
      * - decoding: Set by libavcodec.\
      */\