The definition of Device Health is driven by Indicators, which can be configured at both a global or device level. 

Indicators are analogous to warning lights on a dashboard. Each configured Indicator serves as a single criteria measuring information received from devices (e.g. a telemetry reading) against a specified threshold, with a defined severity: Warning (Yellow) or Attention (Red).

Note that for telemetry that could yield several levels of severity (e.g. moderate vs high Memory usage), multiple indicators are required (one at each severity level). This is shown in the screenshots below. 

Global Indicators

The Global Health Indicators can be accessed through the following button on the Settings Screen:

This screen lists (and allows configuration of) definitions that will, by default, apply to all devices. 

Note that it is possible to override global indicators at the device level (e.g. to apply specific exceptions where needed). 

Device Indicators

The indicators that apply to a specific device can be seen through the following button in the Management tab of the Device Management Screen

This screen shows (and allows configuration of) the indicators that apply to this device, including the status of each indicator. Note that this list includes any Global Indicators that apply to the device - i.e. those that have not been specifically overridden at the device level.