The contact record (for both client and prospects) has a ‘Notes/privacy’ tab:
This section is used to record the ‘opt-in’ status of the contact.
All contact records are set as ‘not opting in’ as default:
How you decide to, or are able to, gain agreement from your contacts to opt-in to communications prior to any direct marketing with them is for you and your company to outline.
Further information on 'opt-in' can be found in our article on this here.