Function rate_desc 

pub fn rate_desc(
    settings: &CaptureSettings,
    fixed: Option<&str>,
    capped: bool,
) -> String

The rate-control field of a video stream’s log line, as the session applies it: a backend with one rate control (fixed, from FrameEncoder::fixed_rate_control) runs it at the bitrate target whatever the mode, and every other session runs its mode, a CRF one under the peak of its bitrate where it is capped (session_capped) and with no bitrate cap otherwise.