This read-write threshold sets the shallower transmit-queue depth at which the port enters mild congestion and begins discarding only red, clearly excess, non-conformant packets, a gentler first line of defense before the harsher severe-threshold discard policy kicks in. An admin sets this below lportSevereThreshold to create a graduated congestion response: mild congestion sheds only excess traffic while amber, discard-eligible but within some tolerance, traffic is still passed. For example, with lportMildThreshold at 100 packets and lportSevereThreshold at 200, a queue depth of 150 would already be dropping red-marked frames from over-subscribing PVCs while still carrying amber-marked frames through.