MFBR
Maximum Flow Bit Rate: the upper limit on the bit rate that a GBR QoS flow is expected to consume, used for admission control and resource reservation.
MFBR sets the upper bound on a GBR flow. Traffic up to the GFBR is guaranteed; traffic between GFBR and MFBR may get through when there's spare capacity, but it isn't protected; anything above MFBR the network is entitled to discard or rate-limit. So the two parameters bracket a GBR flow: a guaranteed floor and a hard ceiling, both specified per direction.
Why bother with a ceiling on a guaranteed flow? Two reasons. It bounds how much the network has to plan for during admission control, and it stops one flow from grabbing resources far beyond what its service actually needs. A codec that occasionally spikes can be allowed some headroom above its guaranteed rate without letting it run away. Enforcement typically sits at the UPF in the downlink and is policed by the UE and RAN in the uplink.
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