Yup, most schools have something like this. You have registered for class already, and then have flexibility to change your schedule during something like the first 2 weeks of class. If you realize you just don’t click with a professor, or something is not what you expected subject-wise, you can drop and add something else. The limits are, there has to be room in the class you want to add, and you have to scramble to catch up for what you missed. My kids have done that, but they usually decide after the first class meeting if they need to make a switch. In larger classes, you might go to the lecture without actually being registered, if you think you might be interested, to see what you think. At a smaller school/class, the professor with an 18 student cap may not take kindly to 10 extra kids showing up just to “see” if they might want to switch into it.