DotNetNuke Roles have "IsPublic" attribute. I wandered what the business meaning of the value. After reading the 4.0.2 code my understanding that if IsPublic=true, user is able to subscribe to the role, if it is free (see DotNetNuke\admin\Security\Register.ascx.vb).
I didn't find any more business logic coded.
It seems that even if public=false, role is visible to user and even available for payed subscription.
(I haven't tested it yet.)