Opt-in management can either be done at a profile level, or at a group level. By default this is set to profile level, meaning that if a customer opts out having received one of your emails, they would be opted out from receiving emails from that profile in future, but would still be able to receive emails you send through other profiles (providing they are still opted in to those). This is particularly useful if you have different brands, and don't want someone to have to be opted out of all of them.
You also have the option to change this to group/account level, meaning that if someone opts out of one profile they are opted out of all profiles under your account. This could be useful if you have multiple profiles within one brand (e.g. you could have a newsletter profile, a sales email etc...) - which would all be classed as marketing for one brand.
To change your Opt-out Management settings please contact your Account Manager