In Canada, a major (also known as a major concentration or concentration) is a term for the specific group of courses that give you a basic knowledge of a field of study, which is in addition to your core curriculum. A major is a term most commonly applied to a program of study leading to a bachelor's degree. The public administration and policy school or program will define a framework for this specialized portion of your studies, including a certain number of required courses and a certain number of "elective" courses relevant to the major. The school will also define your general education or foundation education requirements. Some majors effectively define your full course of study; many others allow you considerable latitude both within your field of study and in their other courses. However, not all public administration schools in Canada have majors, and may offer a program of "general administration" studies, which is, in fact, a major of a different kind.
Although the majority of international students choose their major before entering a university or college, many others will select it during their first or second year of a three- or four-year program. Many schools or programs do not allow you to officially declare your major until the end of your second year so that you experience a broad range of courses that will help you choose the most appropriate major. If you are interested in a major that requires a lot of classes, or classes that are limited to students in that major, you might have to declare earlier than usual (i.e., the fall semester of your second year). As well, for some majors international students may need to take specific courses (pre-requisites) during first and second year before even being considered eligible for upper level courses. Some public administration and policy majors have limited enrolment so in fact you may even be required to apply to get in to the major you want, including attending an interview or writing an essay.
Community college programs in Canada generally do not have majors as such. Their 1- to 3-year diploma and certificate programs carry a designation, for instance Diploma of Public Sector Management, which is much more specific than, say, Bachelor of Arts. Often, students in a community college program--sometimes all the students in the department--will take a common set of core foundational courses in the first year (or term, in the case of shorter programs) which then enables them to pursue a specialized area of study. But generally the field of study at the community college level is career-focused to the point where students do not take "elective" courses as they do for bachelor's degrees. Hence the designation of the diploma effectively stands in for the "major."