At first I was also confused and then realized that these are 2 different options.
One option is the same as enable user registration via mail in other words - if you want to subscribe to the topic you will have to confirm by returning mail and the second is to set the default button to 'yes' without clicking on it.

@Robert , @Martin , please correct me if I'm wrong and I'm wrong I'll second the question.

wpForo has double opt-in subscription system. A double opt-in means that not only does the user have to enter their email and click sign up, they also have to confirm their subscription via a link which send to them in a confirmation email. This is why many people say that the term double opt-in is incorrect, we should be using the term confirmed opt-in instead. However some administrators can say it’s not necessary for registered users, because they have already confirmed their registration so they are owners of their email addresses. In this case we added this option to disable subscription confirmation system.