Imagine a customer in Berlin orders something from your shop. The checkout is in German, the prices are in euros, and the confirmation page says the parcel will arrive "Montag, 6. Juli". Everything you built in the earlier chapters works.
An hour later, the "your order has shipped" email arrives. It's in English, and it says "Monday, July 6, 2026 at 2:00 PM", which is two hours out for someone in Berlin and written in an order they'd never use.
Nothing in the web app broke. The email was sent by a background worker, and the background worker had no idea who it was writing to. It used the app's default locale and the server's timezone, because that's all it had.
Most of this book assumes there's a person on the other end of a web request. A surprising amount of what users actually read is produced when there isn't: emails, scheduled reminders, push notifications, text messages, PDFs, invoices and API responses. This chapter is about all of those.