The value is used by some bitstream to mark the maximum bit rate.

See thread:
Subject: [FFmpeg-devel] [PATCH] Add rc_max_rate to ffprobe stream output
Date: Sun,  1 Jun 2014 17:36:36 +0100
To: ffmpeg-devel@ffmpeg.org

http://thread.gmane.org/gmane.comp.video.ffmpeg.devel/178461Signed-off-by: Stefano Sabatini <stefasab@gmail.com>