Vars plugins inject additional variable data into Ansible runs that did not come from an inventory source, playbook, or command line. Playbook constructs like 'host_vars' and 'group_vars' work using vars plugins.
Vars plugins were partially implented in Ansible 2.0 and rewritten to be fully implemented starting with Ansible 2.4.
The :doc:`host_group_vars <vars/host_group_vars>` plugin shipped with Ansible enables reading variables from :ref:`host_vars` and :ref:`group_vars`.
You can activate a custom vars plugins by either dropping it into a ``vars_plugins`` directory adjacent to your play, inside a role, or by putting it in one of the directory sources configured in :doc:`ansible.cfg <../config>`.