I'm trying to learn how to use this feature, and I'm struggling. Is there any particular docs I should focus on? Or perhaps certain tests that demonstrate the feature well?

What you looking to achieve?

I'm trying to learn how to use this feature, and I'm struggling. Is there any particular docs I should focus on? Or perhaps certain tests that demonstrate the feature well?

Does this link answer your question? https://tox.wiki/en/rewrite/config.html#labels

If not, please create an issue.

@jugmac00, I read that before making this comment, but thank you!

@gaborbernat I want to create a new env that runs in almost the same way as the default env except it has more extras. I'll try to give a minimal example.

[tox]
envlist = py36, py37, py38, py39
minversion = 3.15

[testenv]
description = Invoke pytest to run automated tests
commands =
    pytest \
        --cov \
        --cov-append \
        --junitxml {toxworkdir}/junit.{envname}.xml \
        src/tests
        {posargs}
extras =
    testing
passenv =
    HOME
setenv =
    COVERAGE_FILE = {env:COVERAGE_FILE:{toxworkdir}/.coverage.{envname}}
    TOXINIDIR = {toxinidir}

[testenv:gitlab]
extras =
    gitlab
    testing

I want the gitlab env to do the same thing as the default env (run the tests on py36, py37, etc.), except also install a few more additional dependencies.

EDIT: Sorry, didn't see your updated comment which asked me to create a new issue. I will do so.

Please do not hijack this pull request and create a discussion - thank you.