Hi Cloudbase - hope you're keeping well!
If you physically 'click' on the area need for improvement from the levels page, a small message window pops up with an explanation (screenshot 1). We're working on improving the communications though, and thank you for bringing this up as a point of confusion!
Here's the best explanation for a 'responsive ad' through Ezoic :
Most common use case would be to add a wider ad at the top or the bottom of the page for larger screens and/or to add extra ads to the side of the content for sites that have fixed content widths.
However, this does not mean you want to go through and set a minimum screen size for all the placeholders that already exist.
The publisher would want to create specific placeholders that would serve at certain screen sizes only, and therefore are 'responsive' to the screen size..