The rate, expressed in bytes per second, at which the fair queuing queuing discipline (a.k.a., fq qdisc) - used as a default by Fastly - will attempt to evenly send data to the client when not restricted by either the congestion window or the availability of application data. It changes throughout the lifetime of the connection based on the current size of the congestion window and the current round trip time estimate.
Fair queuing seeks to use a pacing rate a bit higher than is necessary to transmit one window of data in one round trip. Pacing reduces on-path buffering. The rate includes all packet transmissions including TCP and IP overhead as well as acknowledgments and retransmissions.