diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 5a81a379e3..a3bfcbc431 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -658,6 +658,7 @@ typedef struct AVPanScan{
      * is this picture used as reference\
      * The values for this are the same as the MpegEncContext.picture_structure\
      * variable, that is 1->top field, 2->bottom field, 3->frame/both fields.\
+     * Set to 4 for delayed, non-reference frames.\
      * - encoding: unused\
      * - decoding: Set by libavcodec. (before get_buffer() call)).\
      */\