PacketType specifies the type of an RTCP packet
RTCP packet types registered with IANA. See: https://www.iana.org/assignments/rtp-parameters/rtp-parameters.xhtml#rtp-parameters-4
Transport and Payload specific feedback messages overload the count field to act as a message type. those are listed here.
https://tools.ietf.org/html/draft-holmer-rmcat-transport-wide-cc-extensions-01#page-5