Contents in a Listed subject are private to its members only.
However, anyone in the workspace can list this subject and know it exists.
That's where a Secret subject can help:
If no one in the workspace should know a subject exists except its members, you can make it secret.
A secret subject can only be seen by its members as well as its contents of course.
You cannot tag a content to be posted to the workspace nor to any other subject.
How to join a Secret Subject:
Only an Officer of the secret subject can invite people to become members.
Secret subjects are available on Premium workspaces only (Team plan).
Making a subject secret when creating it:
Making an existing Listed subject > Secret :