Hyperscale Page Servers

According to the docs and the image below there are "covering SSD caches" and "non-covering SSD caches". I can't seem to understand the difference between "covering" and "non-covering". Can someone explain this to me please?

https://learn.microsoft.com/en-us/azure/azure-sql/database/media/service-tier-hyperscale/hyperscale-architecture.png?view=azuresql#lightbox

This is the link to the image, as I cannot insert the image directly.