This sets the ceiling, in kbps, on upstream bandwidth an ONU bound to this profile can ever burst up to, even when spare PON capacity is available, per the description and its 512-1000000 kbps range. This cap is what actually enforces a subscriber's purchased tier speed - without it, an assure-and-max profile could let a lower-tier subscriber burst indefinitely whenever the PON happens to be quiet, undermining the point of a tiered service plan. A max rate of 50000 (50 Mbps) means that subscriber's upstream can burst as high as 50 Mbps during quiet periods but never beyond it, regardless of how much idle PON capacity exists.