Course registration
There is no mandatory presence in lectures and exercises at RWTH. Thus, you may walk in any lecture on the campus no matter what subject. Note that in contrast to lectures and exercises for most of the lab courses presence is mandatory.
Consequently, there is no need for course registration from a formal point of view. Even without registration you may attend the courses. The benefit of course registartion lies in the access to the moodle learning rooms in which you are automatically booked upon course registration. This will give you access to the course documents. On the other side the institutes have a clear idea of how many students are listening, so it is strongly recommended to also regisetr for lectures and exercises.
Though there are deadlines for course registration you may ask the institute offering the course to regsiter you after the deadline. This is unproblematic from a formal point of view since only the institute is involved (Note: this is different for Course registration.