As part of the Drupal 8 release cycle, the Layouts API has been introduced in order to provide contributed modules with a unified approach for defining layouts. For example, modules like Panels and Layout Builder make use of this API to define layouts that contain regions and that can render content and all sorts of things inside.
Layouts have been introduced in version 8.3 of Drupal as an experimental module (called Layout Discovery) and marked stable in version 8.4. At the same time, a new experimental module has been introduced, called Layout Builder, which uses this API to provide site builders a way to build layouts for regular content.
We won't be using layouts going forward in this book but it's important you know how to work with them in case you need them. So let's quickly talk about how you, as a module developer, can define and make use of layouts...