IGNOU study centre is the place allocated for counselling sessions. This is the place where you submit your assignments at the end of a semester of a specific course. The study centre approves your assignments and signs the exam form which makes you eligible to sit for the exam.
IGNOU regional centre is just the administration department of your course. Any queries related to ID Card, Books, Exam Form submission, and re-registration can be submitted here. Your examination form along with the prescribed fee is also accepted at RC only.
In simple words, the study centre is the teaching department whereas the regional centre is your administration department.