@ucbalumnus Thank you!
Yes, I’ve come to realize this. UMass Amherst is a good example. CS/Engineering/Business majors at UMass Amherst are restricted, i.e. students must apply to those majors first before being put on exploratory tracks. A general ‘undecided’ student would not be able to apply after enrollment.
Not all schools are like this though. E.g. Undecided can declare CS or Engineering as long as they can achieve minimum GPA requirements in the required core courses.