Universities offer courses in a wide variety of disciplines. At some point, generally by the end of the second year, students are required to declare a major. In some cases, students begin classes in a program of study from the minute they enter the university.

Some college freshmen seem to have been called to a profession at an early age, 

and their determination to become a teacher, or doctor, or engineer has never wavered.

请问文中的declare怎么理解？