What are the concepts of compulsory courses and elective courses in universities? You can't choose another subject if you choose one subject?

It depends on which university, for example, my university, compulsory courses are compulsory, and elective courses are chosen according to my own time schedule and hobbies. The school only stipulates that elective courses should be completed for more than 30 credits in four years (our school). Elective courses are divided into several types, and then it is stipulated how many credits each type must complete before graduation.

Elective courses in our school are divided into general courses, cultural quality courses and interdisciplinary elective courses. Each semester, everyone can choose at most one course or not. This is a random lottery system. So it's not that you can't choose other subjects if you choose one subject, but you can choose at most one subject. If you have enough time and are lucky enough, you may choose three courses a semester (take our school as an example, I hope you can understand).