Glossary
Site Configuration
The configuration that assigns a page tree its domain, its languages and its URL rules.
What it contains
The site configuration ties a page tree to everything needed to serve it: the domain it is reachable under, the available languages with their identifiers and base paths, the rules for building URLs, and the pages delivered when something goes wrong.
For multilingual sites it is the place where you decide whether languages run in directories or on separate domains, and what happens when a translation is missing.
Routing
The routing configuration lets you define how addresses are built, including for parameters coming from extensions. Instead of an address with parameters appended, you get a readable path.
That is more than cosmetics: addresses like that are more stable, easier to understand and less ambiguous for search engines. What matters is settling them at the start, because every later change drags redirects behind it.
Error pages
The site configuration also records what happens when content is not found. The sensible answer is a page of your own in the site’s layout that leads somewhere, rather than a blank default message.
With several languages, that page should exist in each of them. A German error message inside an English language version looks unfinished, and that is exactly the moment it becomes visible.