Reachability is whether we can talk to the device.
Reachability will return 100 if can talk to the device, else it will return 0 if not.
Availability is the snmpcollector interpretation as to whether the device is actually down. This is based on the sysUpTime.
Availability will return nothing if the snmpcollector cannot talk to the device. If the snmpcollector can reach the device it will query the sysUpTime.
If it appears to have been up when we couldn’t talk to it, we return 100 (it was up, just not reachable), if it wasn’t up (sysUpTime seems to small), we’ll return 0 (it wasn’t reachable, and was actually down).