[ansible-project] Add conditional logic to variable files?

I've been using Ansible Galaxy roles from Jeff Geerling. In his Github READMEs he seems to say that you can override variable settings in his default/main.yml and vars/*.yml files by reproducing the desired settings in a /vars/main.yml file and then re-assigning them your desire values. This

[ansible-project] Meaning of Variable Name Starting With "__"?

I'm about to use geerlingguy.apache and saw this code in the main.yml task: - name: Define apache_packages. set_fact: apache_packages: "{{ __apache_packages | list }}" when: apache_packages is not defined What does this double underscore mean? __apache_packages I know that double under