Two group addresses can be configured for a CO2 sensor. CO2 detected and CO2 level. Both are optional, but you'll need to configure at least one to make this useful. When configuring CO2 detected you can receive notifications in HomeKit when levels are abnormal. You can create automations based on this notification in your HomeKit configuration, it can automatically turn on a fan for example.
Note: This sensor is not available in Alexa and Google.