The Ansible 2.8 documentation about tags includes a section about tag inheritance, explaining how it is that tags are inherited when using include_tasks vs import_tasks. A little more explanation is also given in John Wadleigh’s post.

Sometimes you may want to make use of a tag in a set of included tasks or an included role and, for whatever reason, you want to avoid importing the tasks/role. This is possible - see the illustration below.

---
#
# playbook.yml
#
- hosts: localhost
  connection: local
  gather_facts: no
  tasks:
    - include_tasks: tasks.yml
      tags: always

#
# tasks.yml
#
---
- debug:
    msg: "tag1"
  tags: tag1

- debug:
    msg: "tag2"
  tags: tag2

#
# output of: ansible-playbook playbook.yml --tags tag1
#
PLAY [localhost] **************************************************************

TASK [include_tasks] **********************************************************
included: /tmp/examples/tasks.yml for localhost

TASK [debug] ******************************************************************
ok: [localhost] => {}

MSG:

tag1

PLAY RECAP ********************************************************************
localhost : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0

What we can see happening here is that when the pre-processing occurrs, the always tag ensures that the include_tasks task is included at run-time, which then provides access to tag1. Without the always tag, none of the included tasks will be processed at all.

A side-effect of this method is that using the --list-tags argument will not yield any of the tags from included tasks or roles.