Assuming that you know views and you have already created a view of nodes which you wish to "tag" with specific node terms. If necessary add the filters to the view to narrow down the exact nodes. Make sure to add permissions to the view so that only administrators can access it in the case of sensitive data.
Now we add a new "field" to the view "Bulk operations: Content." Activate "Modify entity values". Then select the taxonomy term reference field which you'd like to use.
Aforementioned will render the operations drop down and a checkbox to select all or specific nodes where you wish to apply tags.
Select all the nodes which you'd like to tag and execute the operation "Modify entity value". Now we can add the tags which we wish to apply by either appending them to the existing ones "Add new value(s) to Tags" or overwrite each node with your term(s). It's recommended to insert the terms to the vocabulary beforehand to avoid VBO creating the same term multiple times.
Press Next and confirm your operation to apply the terms to the node. If you have any other questions feel free to comment below!