It's used in
RTCRtpReceiver.getParameters(). Its also use used when calling
RTCRtpTransceiver.setCodecPreferences() to configure a transceiver's codecs before beginning the offer/answer process to establish a WebRTC peer connection.
Most of the fields in this property take values which are defined and maintained by the Internet Assigned Numbers Authority (IANA). References to relevant IANA documents are provided in the see also section at the end of this article.