Hi Orkhan, An Opt-In is basically a clients Express or Implied Consent received through some way shape or form that conveys that they are willing to receive further electronic communication from you (For example- newsletters and promotional emails from an organization).
Now for a parent organization that has 4 or 5 brands, they might 4 or 5 corresponding websites and it is important for the organization to know which brand's newsletters somebody has subscribed to. For example if they've subscribed to Brand 'A's' newsletters, they cannot received newsletters about Brand 'B'. My understanding is that this is not available as a feature today and that it will have to built custom. Can you please confirm?