This Default Field Type is available in Profile Builder 2.0 free version.
The Default – Username field is a mandatory field for the Registration and Login forms. This field cannot be deleted from Manage Fields and it is marked as Required by default. Inside the field options you can set the field title, description and default value.
Usernames are unique and cannot be changed.
This is how the Default – Username field we created above looks like in the front-end.