This read-write value, expressed in 256-packet increments up to 4096, sets the falling threshold for how many transmit packets are allowed to sit in this port's buffer before congestion control eases, and it is DURABLE so it survives a reboot. Admins tune it to control how aggressively a port pushes back on senders once its transmit buffer starts draining. For example, lowering the ceiling on a low-priority port makes it declare non-congested sooner, freeing shared buffer memory for other ports.