Sometimes, testing isn't only about what is in the ad, but also what isn't.
Maybe you want to test an ad version that has a logo, and then doesn't. Or features a background, but then doesn't.
In this case, it's an "empty variant," meaning, it's a variant version that doesn't display anything, versus the other versions which do.
You can create an empty variant version when you're in Variant Mode.
In the Variant group, click on "add a new variant" at the bottom of the group:
It will create a blank variant.
It will still show a command to drop your variant, but you can ignore that, and when your ads generate, it will simply be a blank version of that spot.
If you have an existing variant that you want to make blank, just click on the little edit icon to the right of the variant name and select "remove variant." It will remove the variant, but still leave the "slot" for it there:
Here's a quick overview animation of all these functions: