There are four types of Subjects: the Dynamic, Open, Private and Confidential subjects. Here are the main differences

Dynamic subjects include all of the workspace members at all times. It automatically adds and removes members as they arrive and leave the workspace.

The content is accessible by all members and everyone can create and interact with it. Everyone will see the activity updates on their own Activity timeline.

Dynamic Subjects are represented by an empty house icon.

Open Subjects are visible to everyone in a workspace, member or not. They are listed in the Subject index and search results. Anyone can become a member of the Subject by clicking the “Join as a member” button on the top right corner.

The content is visible by all workspace members, and everyone can create and interact with the content. However, only the Subject members will see the activity updates in their activity timeline.

Open Subjects are represented by an empty circle icon.
 

The Private Subjects are visible by all workspace team members in the Subject Index or through a quick search. Non-members can ask to join Private Subjects by sending a request or can be invited by current members.

The content of a Private Subject can only be seen by Subject members when shared within the specific Subject, but can also be seen by other people if shared on other Subjects.

Private Subjects are represented by a full circle icon.
 

The Confidential Subjects are invisible to non-members, therefore no request to join is possible and new members can only be invited in by current members.

The content of the Subject (messages, tasks, or document) can only be seen by the Subject members, and cannot be shared on other Subjects.

Confidential Subjects are represented by a padlock.
 

 
There is also the General Subject, which is unique for every workspace and that is essentially an open subject with every workspace members as subject members at all times. Learn more about the General Subject.