You can't choose this course when the class capacity is full, because generally speaking, the number of teaching classes is limited in order to ensure the classroom order and teaching effect. At this time, you can re-select other unfinished courses.
In the pre-selection stage, if the number of selected students exceeds the course capacity of this course, all students who choose this course need to draw lots in the primary selection stage, and only the selected students will be regarded as selected students.