Betterteam offers a simple way of categorizing candidates by clicking on the "Like" or "Reject" buttons or choosing one of the many Statuses from the dropdown when viewing the candidate.

Only you can see the Status you give a candidate. No notification is sent to them when you click Like or Reject, or when you choose another Status in the dropdown.

If you want to add more detailed feedback about the candidate, you can save it as a candidate note that only you can see.