This value reports the port type as dod-ip, arp, or snmp, and is explicitly not redundant with dlType since a single data link layer service can support multiple port types, such as LLC1 hosting both llc1 and snap ports. An administrator uses this to determine exactly which upper-layer protocol a given port entry serves, which is necessary since the parent data link layer type alone doesn't tell you that. For example, on an LLC1 data link layer with several port entries, the admin uses this field to separate the ARP-handling ports from the DoD IP-handling ports.