What are they, in plain terms? Is it just manually hoisting what you know a route needs all the way to the top of the tree? Or is it some compile time magic that analyzes your components used, making the decision for you on what to fetch for a route?
And Meta's way of doing this is with Relay, so you are still defining component data requirements with the components and query fragments, but there's a compile step that produces those route level queries.. so you still get "co-location of a component's data requirements with the component", and "top level early data fetch" for the render-as-you-fetch pattern.
This change breaks the fetch-as-you-render pattern where components make individual data requests for their data, because that pattern is considered bad for performance (for Meta's use case).
Essentially data fetching starts as soon as possible. Right after routing, right before rendering.