This is for Detailed group registrations only where a form is given for member 1, then member 2, etc then ending with the billing page. You can set the field (in a group registration) to be used for each member, or only on the final billing page, or on BOTH.
This is explained in video tutorials too.
So, say you need the user to select if they are registering as an adult or a student... you'd want that for each member of the group registration. If the field only pertains to the group as a whole, then just assign it to the billing page.
For a future version it would be great if we had the option to turn off a lot of those options, including the whole group side of things. It makes the whole interface much more complicated than it needs to be, which in turn makes the learning curve steeper for clients who really just need a simple reg system...