Good codebase is easily modifiable. Diff Challenge verifies how easy is to add a new feature to your application or change its existing behavior. This process is done in two steps.
In the first step you write code according to a provided specification.
In the second step, you receive a new version of the spec, which contains some new requirements. If your application meets these new requirements, it means that you created a diff which made it possible. Diff Challenge is about evaluating quality of this diff.