Availability Report Questions

[js1]

In in the Zabbix doc:

21.1.1.Monitoring of server's availability
At least three methods (or combination of all methods) may be used in order to
monitor availability of a server.
ICMP ping (Key "icmpping")
Key "status"
Trigger function nodata() for monitoring availability of hosts using only active
checks

Suppose I want to use a custom external script to monitor the status of a server or template, does that mean when I go to create the Item, I use "status" as the name of Key?

Also, how are the Availability Report Graphs generated? Specifically, what does the True, False, and Unknown columns mean?

Thanks for any additional insight.