Can anyone help me understand this mysterious concept!
As we can see the below options within Use email attribute from Contacts
I could NOT find this "Email Attributes > Email Addresses" neither in ATTRIBUTE GROUP nor in POPULATIONS!
It is the SYSTEM_DATA Attribute Group in the DataDesigner.
When you watch the All-Subscriber-Table inside marketing cloud this is the stuff where this data refers to. It is the email address that is attached to the subscriber in the marketing cloud. All e-mails where you only include a subscriberkey to send an email will result to send in this direction (normal send proccesses etc.).
These email-adresses can only be changed via import-file processes or some manual tasks like changing it in the subscriber search (and i believe some rest or soap calls, but not sure on the that one).