Topic:

How exactly does the Opt-out feature work?

I thought it would remove that person just from that mailing list but it seems to remove that email address from the entire account. Or does it just remove them from anything associated with that event?

Solution:

The only thing Opt-Out should do is mark the person's profile on the Mailing List as "opt-out", showing that they do not want to receive email jobs.  It should do nothing to their registration, and it should not remove them from anything. 

If your new mailing list is a filtered list, then the opt-out is attached to the profile and a new filtered list is just grabbing a new bunch of profiles from the Master list.
 
If your new mailing list is a custom list, then that means you uploaded new profiles, so you may end up with more than one profile for the same email address, and the opt-out will only be attached to the old one...

Additional Information:

When you upload a new list to an account and want to maintain the OLD opt out preference for a record already in the system, then you will need to include an OptOut field/column on the list with the appropriate response selected.

Related Links:

How do I upload a mailing list?
http://forums.regonline.com/forums/post/248.aspx 

What is the Master List, and what is it used for?
http://forums.regonline.com/forums/post/623.aspx