Let say you have tests that trigger real HTTP calls (e.g.: contracts). And you want those to be excluded from the default test run.

# test/lib/twitter_test.exs
defmodule MyApp.Twitter.HTTPTest do
  use ExUnit.case, async: true

  @moduletag :twitter_api

  # ...

# test/test_helper.exs
> ...
ExUnit.configure exclude: [:twitter_api]  # exclude this tagged test from default test run

In order to run thest with the twitter_api tag:

mix test --include twitter_api

To run just these tagged tests:

mix test --only twitter_api