There are two ways to mark records as sensitive.
Firstly, you can select records in search results, and then select change record visibility in the selected record actions dropdown box, and then click apply change to selected records.
Secondly, you can go into an individual record by clicking on the View Records icon in the search results
Then go into the Admin tab, and then tick the Hide sensitive details for this records checkbox.
Once you have marked a record as sensitive with the Hide sensitive details for this records checkbox, you can switch your view to least in the top right hand corner of your screen, allowing you to browse records with the minimal permissions, and with sensitive records hidden.
PLEASE NOTE: It will take up to 24 hours for changes to the sensitivity of records to be reflected in the DDb.