Access tab controls access to the plan. It does not control access to your content. It's so you can have a plan accessible to specific usergroups only. How you user it or whether you need it is entirely dependent on your site.

If you want to protect articles by view access level you don't need CBSubs to do that and instead just set the view access level for the article and/or its category.

To protect article access using CBSubs you need to have CBSubs Content installed then ensure the article has completely public access. Now either select the category, the article, or protect its URL directly within your plan configuration under Integrations > Content.