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?