If you would like to apply a tag to an entire (email) conversation thread, complete the following:
You will see the checkbox necessary to apply your tag change to the entire conversation, named accordingly, relative to the document you are viewing.
In order for these changes to be applied, the checkbox must be selected PRIOR to your tag addition.
Once the box has been selected, you can select the necessary tag and it will be applied to all the conversations related to your current document.
*Please note, this information holds true for all matters created before 12/06/2016. If you have a matter created on or after 12/06/2016, please read 'Take Full Control Over How Your Tags Propagate In DISCO' for a detailed explanation on how to apply a tag to an entire conversation.