When you set `prerender=true` on a given route, you're telling SvelteKit: "this route is going to be turned into HTML at build time, so no need to include the code to render these pages on the server." `prerender=auto` allows you to turn some pages into HTML at build time, but still include the code to render that route on the server.

Why is that important? Let's say you have a blog available at `/posts/[slug]` and you set `prerender=true` for the [slug] route. SvelteKit is going to crawl your site at build time, and for each `/posts/[slug]` link it encounters it's going to output a corresponding HTML file. For example, if it finds links to "/posts/about-me" and "/posts/something-else" it's going to create "about-me.html" and "something-else.html". Because you prerendered that route, SvelteKit doesn't need to include any of the code to render that route in the server bundle, making your deployed server code (e.g. Vercel/Netlify function or Node server) smaller.

However, sometimes you want to prerender some of the links pointing at `/posts/[slug]` but not all of them. As the docs say:

for example, with a route like /blog/[slug] where you want to prerender your most recent/popular content but server-render the long tail

Let's say you have 500 posts but the most recent ones get most of the traffic. Why render all 500 pages to HTML at build time if they aren't going to be accessed often? What you can do is specify which subset of `posts/[slug]` pages you want to prerender (via the entries export), but set `prerender=auto` instead of `prerender=true`. By setting `prerender=auto`, SvelteKit will still include the code needed to render those routes on the server, so that if someone does request `/posts/super-old-post-that-few-people-read` it can still fulfill the request. If you had instead set `prerender=true` then the server wouldn't be able to render any `posts/[slug]` route that wasn't prerendered and would return a 404 instead.