Email Newsletter is a system-created list (marked so it cannot be renamed or deleted) that Sprwt fills automatically from your website's newsletter signup form / popup. When a visitor submits the form:

• if the email belongs to a registered customer, that customer is added to the list as a Registered User;
• otherwise a new Contact is created (name + email, Receive Email on) and added as an Uploaded Contact.

The signup also raises the User: New Newsletter event, so you can build a flow on that trigger (e.g. a welcome email with a discount). Target the list in campaigns by choosing Email Newsletter under the Lists category. The popup form itself is configured under Website settings (see the popup forms article).