In the pyproject.toml configuration file of Poetry you can specify both dependencies and dev-dependencies, but the documentation does not state clearly what the difference is.
I guess from the name the dev-dependencies will not be installed by a release build, but I didn't notice any difference. How do you use these fields correctly, for example exclude the dev-dependencies in a build?
Your assumption is right. The best use case for dev-dependencies is when you creating a library with optional dependencies. For instance, you are developing ORM which should work with MySQL, PostgreSQL, etc. You have to test that your code is working with all of these RDBMS. You put it into dev-dependencies. But for one, who installs your library, these dependencies are optional and they wouldn't be installed automatically.
Commonly, all libraries that are used for testing or building your application are presented at dev-dependencies.
How do you use these fields correctly, for example exclude the dev-dependencies in a build?
poetry install has a no-dev flag for exactly that scenario.