I may be incorrect - but the way I read that is to say

“If you’re on this list - you may self-register. If you’re not on this list - you may not self-register.”

In other words - you are blocking them by not putting them on the list.

Leaving it blank means there are no restrictions.

I suppose the idea is to have a smaller list by just putting a couple in the block list rather than potentially thousands in the restrict list.

Agree this field seems to be set-up in reverse...we’ve run into the same issue where it’s not about ‘allowing’ certain domains but rather NOT allowing certain domains like @hotmail.com, @yahoo.ca, etc.. Trying to add all allowed domains when you’re audience is Canada-US wide is impossible.