Have you seen code reviews where the code being added is not yet used anywhere in a web service? It's not yet covered with automated tests? It will likely be refactored or deleted in a subsequent pull request (aka PR)? Is a library that is not or only partially used? All of these cases represent non-functional increments to a web service.
A functional increment contains only code that is being used by the service to achieve some functionality. Such code is typically fully covered with automated tests. A functional increment should also be self-contained in a way that it doesn't rely on future changes to expose functionality.