Headers are used to add information to an email. There are standardized headers such as "From" and "Subject", but you can also use so called "x-headers".
In Marketing Suite you can find all of these headers at the top of the Template Editor when creating an email. In Publisher you can find the headers at the top of the email when creating a new mailing under the E-mailings menu.
The BCC (Blind Carbon Copy) can be used to send copies of emails to one or multiple addresses, without showing the recipients the addresses of the others. This reduces the chances of addresses ending up in the hands of spammers, or receiving long chains of "Reply all" emails. Additionally it protects your users from viruses that spread by sending to other email adresses.
X-headers are additional headers that can be added next to the usual standardized headers such as the "From" address and the "Subject". The "X-" stands for eXperimental or eXtension. You can add your own values to add more information to the emails you send. You can name and use these X-headers however you want, for example to add the name of the selection and campaign to each email to use in your own statistics.
With Copernica you can also use Smarty personalization in these headers. You can add a header named "X-PF-ID" for example and use it to store the ID of the profile from your own database for example, because you usually sync it to Copernica. You can then use this ID when your email bounces to look up the contact in your own database and contact them for the right email address.
The list-unsubscribe header is additional information added to an email, containing instructions to display an unsubscribe button with the e-mail. When someone uses the button, a message goes back to Copernica so that the unsubscribe can be processed. This uses the set unsubscribe-behavior on the destination.
In the advanced headers of your template you will find the option 'Unsubscribe header'. This header is on by default because it is important for good sender reputation. In addition, spam filters are positively affected if they see a list-unsubscribe header with your email, making it more easily pass your e-mail through to the inbox. It is a kind of guarantee that you are are a legitimate sender. And it makes it easier to monitor your reputation monitor for the watchdogs of e-mail traffic (Return Path, Lashback, Listbox , et al).
The list-unsubscribe header is not a replacement for your own unsubscribe link in the template or doument.