SCOM has Monitors and Rules and from the outside they look pretty much the same, but there are some important differences.
Monitors are used for checking the Health State of a component and only store state changes and Alerts in the Operations Database. Monitors do NOT store data in the Data Warehouse.
Rules are used for collecting performance data or to run scripts or for soem reason if you want to generate an alert for a condition but DO NOT want the health of an object affected. Rules store their data in the Operations Database and the Data Warehouse so it can be reported on. You won’t see Rules in the Health Explorer because the Health Explorer only shows state changes which rules are not used for.