By definition, a perimeter is the outer boundary, and securing that perimeter means protecting that area. Everything has a perimeter, from a large university campus to your own personal space on that campus. Law enforcement and security personnel have predefined outer perimeters that keep people either in or out in the case of an event. But you should also look within the perimeter to criticalities that could present problems.
Engineering labs storing equipment or chemicals, on-campus hospitals, dormitories and others all have boundaries of concern and importance. Known as sub-perimeters within the main perimeter, they fall into three main categories: those that require general security, those where processes must be secured, and those where security is employed for safety purposes.