I don't think it would be any of those particular criteria, Tayyab. I think it would just be Zabbix receiving the SNMP data for a given time period. If you wanted a mechanism for system availability, I imagine you could test availability of the HTTPS 443 port. That's often a poor proxy for system availability, so to get true availability, you would need to poll some valid endpoint that will return data.
Take a look through the NBI help pages and you might want to poll something like https://yourPiServer/webacs/api/v1/data/ApiHealthRecords.