Whenever you want an ad-free episode. Which you might want if you have sensitive content that would make ad breaks inappropriate, or if you have a shorter episode, etc. It is up to your discretion— just remember that it affects your ad sales.
This tag is added by clicking the Ad-Free button:
Which is on the Episode’s ‘Basic Info’ tab. Click it, and Dovetail is told not to insert any ads into that episode for as long as that tag is in the Categories field. When using the ‘Ad-Free’ button, be sure to tell the ad operations department email@example.com, so that they’re able to factor it in to sales goals.