StatusPage.io lets you add a component for each of the separate infrastructure or functional part of your service. The best way to explain this is to show you a few of our customers who are using this feature correctly. Check out these StatusPages: Customer.io, New Relic, Vimeo, and KISSmetrics.
To communicate the state of each component, they always have one of the 5 following statuses:
You can change the state of your components from the Dashboard.
Operational means exactly what it sounds like. The component is functioning as expected and in a timely manner.
Under Maintenance means exactly what it sounds like. The component is currently being worked on.
Degraded Performance means the component is working but is slow or otherwise impacted in a minor way. An example of this would be if you were experiencing an unusually high amount of traffic and the component was taking longer to perform its job than normal.
Components should be set to Partial Outage when they are completely broken for a subset of customers. An example of this would be if some subset of customer's data lived in a specific data center that was down. The component might be broken for that subset of customers but is working for the rest and thus there is a Partial Outage.
Components should be set to Major Outage when they are completely unavailable.
Unfortunately, the answer is "it depends". A good rule to use is to add a component for each major functional/architectural division of your service. Refer to the examples above as they do an awesome job splitting their service/product out into components that make sense to their customers.