If you study 100%, this means that your study pace is full-time, which is 30 credits per semester.
If you study less than 100%, this means that your study pace is part-time which is less than 30 credits per term.
You can choose what percentage of full-time (30 credits) you which to study.
You should select 'All' if you don't mind whether the course is full-time or part-time.