By default, all posts you publish are accessible to anyone on your team, unless the post is part of a topic with restricted access.
To adjust the accessibility of all the posts in a topic, open the menu in the upper right-hand corner of the topic page and select Edit topic. Under Who can access, you can choose Open or Private.
Any setting you choose here will not affect the external accessibility of your posts.
By default, posts are not accessible outside your team. To make a post externally accessible, just click Share in the top-right corner and toggle Create a public link.
Anyone with the link from outside of your team will be able to view the post as read-only.