In Moodle what controls the assignment of role to a new subscriber?

Please answer this if you can, and then retag it. Thanks

To clarify the question, what controls the assignment of role to a new subscriber (e.g. why are some people assigned the role of Teacher in some sites, and others the role of Course Creator in other sites)?

This question came from Annelie ChapmanChapman, so let her know when you answer it.