From OpenNMS

Implements part of a virtual poller, the other half of which is the passive status monitor, which is a poller monitor (like ICMP, DHCP, etc.). Hosts that have passively monitored services default to those services being "up,", and this status can be changed (up or down) with events sent to this daemon. This can be useful when you have another monitoring system that monitors a host, and it can report status to OpenNMS via events.