This trap-only integer identifies the kind of health probe involved in an SLB event, with values icmp(1), tcp(2), udp(3), or http(4). The description notes the switch supports these four probe types, so this field tells an admin what protocol-level check failed or succeeded for a group member. A trap reporting hwProbeType http(4) alongside a member going out of service tells the admin the failure was detected via an HTTP health check rather than a basic ICMP ping.