Need clarification on the difference between Subscriber Role and User Registered Role.?

Why is it when I create a user (in this case just a simple test user) and DNN assigns this user 2 roles: Subscriber and Registered User?

I understand why it assigns it Registered User but why Subscriber? This site I am working has nothing to do with charging users subscriber fees? Also, Why is the Subscriber Role set as Public (default) and not the Registered User role? Should'nt Registered User be set to public?  I un-checked the 'Auto Assignment' checkbox for the Subscriber Role thinking that would enable the Auto checkbox to become enabled? But it didn't

Thanks!

Thanks.

Would you be able to assist me with how I could allow the user to type in their email address as their usersname? First of all I need to increase the password length and all non-numeric characters.