Ansible variables and `gather_subset`

Question

How do I find which ansible variable belong to which gather_subset group (https://docs.ansible.com/ansible/latest/collections/ansible/builtin/setup_module.html#parameter-gather_subset)?

Following an example to get ansible_hostname. For which I basically did trial and error, but I still don't know if system is the only subset which includes ansible_hostname and moreover if it is the minimal subset.

- name: Test gather facts
  gather_facts: true
  gather_subset: ["system", "!all", "!min"]
  tasks:
    - name: Test delegation
        - name: Debug
          ansible.builtin.debug:
            msg:
              - "{{ ansible_facts }}"
              - "{{ ansible_hostname }}"

of course I could also use a task like

    - name: Filter and return only selected facts
      ansible.builtin.setup:
        filter:
          - hostname

to get only the variable ansible_hostname. The thing I don't like here is that it needs to be done as a task and not globally next to gather_facts.