This read-only integer is simply the physical port number that a rate-limiting table row applies to, letting the ingress/egress limit settings be tied to the correct interface. An admin relies on it to confirm which port they are configuring before changing a rate limit, since the rate-limiting table can hold one row per port. For example, seeing rateLimitingPortNum report 5 confirms that the ingress and egress rate values being read alongside it belong to port 5, not some other port on the ES8510.