In addition to the ability to assign a tag to a recipient when they fail a phishing test, you can also assign a tag to a recipient when they have just been sent a simulated phishing email (I.e., they don't even have to fail the test!).
While the use-cases for this are many, one of the most popular uses is to automatically phish recipients that have not yet been phished.
To accomplish this, all you have to do is create a repeating simulated phishing campaign (see How Do I Create a New Phishing Campaign for more info) which excludes and associates the same Tag during Step 2.
See the screenshot below:
Exclude and associate the same Tag
As you can see in the above image, this campaign will send to All Recipients, excluding those with the "Phished" Tag and it will assign the "Phished" tag as they are phished.
Set the campaign to repeat
Using this example, all recipients will be assigned the "Phished" Tag during the first sending event, then each subsequent monthly sending event will only go to those recipients without this Tag. As such, only new unphished recipients will be included in each monthly send.