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.
You can also use the list unsubscribe header. This is used to display the unsubscribe link right on the top of the email in some email clients. We recommend always using at least an unsubscribe link. This will help your sender reputation and you can set unsubscribe behaviour yourself.