We are building a (Zabbix-based) monitoring system for our applications; hovewer, I'm having difficulties in defining what to monitor?

I have so far come up with the following general categories:

• hardware data: cpu, ram, swap, etc.
• middleware data: perfomance/health for MySQL instantces, Tomcat instances, JVMs, etc.
• logical or application data: the current status/health of the system, e.g. number of active users, page request, etc.
• kpi data: data for business, e.g. user registration over time.
• dashboard: quick overview of the system (e.g. microservices are running or not).

Are there any other fundamental categories for to monitor? Or is there another category system to use?

UPDATE: the purpose of the monitoring is

• the see if the system functions correctly (at high-level, e.g. no services are down, etc. - much like a smoke-test)
• see, if there are any indicators, that the system is likely to crash (e.g. historical data predicts that we will run out of disk space)
• if any of these occur, send a warning to the appropriate staff (e.g. via e-mail)