@Mariposista
Usually you have some core compulsory modules, about 60 - 90 credits.
Then you have a choice of modules to study for the remaining 30-60 credits to total 120.
The reason for this is to give students some experience in areas of interest, to widen their scope to change to a different degree program at a later point or to do a module that might be helpful when applying for postgrad/work placement/employment later.
Students are automatically enrolled and have a place on the modules that are compulsory for their degree.
It is then up to the student to log on, look at the choices for the remaining credits and enrol in a timely fashion.
Sometimes the optional courses book up quickly, either because they are compulsory for another course or because they are really popular (trendy topic, or a lecturer who is known to be a) good or b) to hand out top marks like sweets)
People can hit problems where they leave signing up to courses to the last minute or where they really want to do specific modules where the timetables clash.
The latter happened to me a few times, you have to prioritise which is more important and take a different class that fits with the timetable. Or you negotiate being on both classes and deal with it that you won’t be able to attend all lectures.
I know people who did both, most opted to change from their initial choices to have a workable time table.