Device Availability in OpManager is based on the ICMP Ping response of the respective devices from the OpManager server. The Availability, Packet loss and Response time would be calculated based on this response.
The basic criteria is that the devices should be reachable from the OpManager server to go ahead with the discovery, else the device might fall under the "Not Monitored" state.
The devices can be still reached when the ping utility has been enabled and it would update once when the device is reachable from the OpManager server.
It is applicable to all server, network devices, firewall, UPS and printers and whatever device type and series that has been added into the application.