If you're using Confluence as a knowledge base for Jira Service Desk, you can choose to allow all active users and customers (that is, logged in users who do not have a Confluence license) to view specific spaces. This can only be turned on via Jira Service Desk.

When a space is accessible to all active users, you'll see the following message in the space permission screen.

Confluence unlicenced users

(info) This permission overrides all existing space permissions, so any logged in Confluence user will also be able to see the space (regardless of their group membership). 

You can edit this permission at any time to revoke access to a space, but it can only be re-enabled from Jira Service Desk. 

Active users who don't hold a Confluence license have very limited access to Confluence. They can view pages, but can't like, comment, edit, view the dashboard, use the space directory, see user profiles or search your full site. 

See Use Jira and Confluence together  for more information about Jira Service Desk integration.