diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index b635ae7cf31d0f6ef77ff030ad151660245b1f4a..166fdbb2286a11434040276a91d972109ef7813d 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -809,7 +809,10 @@ typedef struct AVStream {
     unsigned int index_entries_allocated_size;
 
     /**
-     * flag to indicate that probing is requested
+     * stream probing state
+     * -1   -> probing finished
+     *  0   -> no probing requested
+     * rest -> perform probing with request_probe being the minimum score to accept.
      * NOT PART OF PUBLIC API
      */
     int request_probe;