it is not a host definable fact, it is a 'host variable' which you normally define in inventory.
When Ansible runs on a host, a JSON object with Facts is returned to the Controller. The Controller uses these facts for various housekeeping purposes. Some facts have special meaning, like the fact "ansible_python_interpreter" and "ansible_connection".
So while you may normally define it in the inventory, it sounds like, from the advisory, it can also be defined by the host.