ClassroomParent mimics the school year, in that you can have a different list of students/parents for each year. It also allows site administrators to begin constructing the new school year before the end of the old year. However, only site admins can access this future year.
When a family is added to the year, they are allowed to perform certain actions, such as completing their contact information using the parent invitation functionality, or the students without parent forms.
They are not able to register as a user until a site admin updates the system to set the new year to be the current year using the promote process. At that point, the parent would be able to register.