The docs are super unclear, and written for those not familiar with ActivityPub.

In future, we hope to develop a deeper integration between your social web profile and followers <> and your public website and registered members. To start with, though, they operate independently from one another.

Your social web profile and followers are separate to the rest of your site and memberships, so you can think of it as a new, additional distribution channel.

Emphasis mine. I think the setup is basically federating links in Mastodon style. So if you have a post available as a subscription, it will prompt a login after clicking through.