There are three levels of privacy:

By default, all contacts are public and searchable by any team member.

Private Contacts

Only the client team will have access to that private contact and their work. Colleagues who don’t have access will see the client and work names, but will not be able to view details or any communication that occurs.

Additionally, you can grant access to a private or hidden work item to anyone outside of the client team.

Hidden Contacts

While hidden items have many of the same rules as private, only the client team will have access to the contact. Colleagues without access will not know the contact or work related to it exists in Intuit Practice Management powered by Karbon (it will not show up in searches.)

If you are assigned a task, note or email on a hidden work item, only those with access to that work item will be able to see those tasks in your to-do list. Scheduled work for a hidden client will also be hidden.

Further reading:
Overview of Privacy
Video overviews of Privacy
Private work