clarify difference from Registration Role?

Yes, they are different. Registration role lets the user decide which role they should join when the register on the site.

This module allows the admin to create a default role that all users are added to.

I'm changing the status to a task for better documentation since you asking the question means that the documentation is a little lacking ;)

Actually...the description in comment #1 isn't accurate.

This module and registration_role are actually the same, as far as I can tell. Role Signup is the module that allows a user to choose a role on login, however Registration Role is actually attempting the same thing as this module.

The main difference between the two modules is the number of roles allowed to be set for each user. This one is specific to a single role as selected by a select box. The other uses radio buttons. In addition, this one sets the roles when the user actually logins vs when the account is created (if I understand the code I read correctly). As far as I can tell http://drupal.org/project/rolesignup is no longer maintained. Thanks for pointing out the similar projects ... was not aware of their existence, perhaps the projects can be unified.