This read-create setting selects which protocol's packets are considered for rate-limit filtering on the port, choosing between icmp(1) and all(2), and defaults to icmp. It lets an admin scope the DOS rate limiter narrowly to ICMP floods (a common ping-flood attack vector) or broaden it to cover all traffic types. An admin seeing a UDP-based flood, not just ICMP, would change this from the default icmp(1) to all(2) to cover the wider attack.