For graduate school a statement of purpose for the admissions process is generally a requirement. There are, however, undergraduate schools that ask for a SOP.
The statement of purpose question can appear in a variety of forms. The University of Pennsylvania, has a 450-word SOP: “Considering the specific undergraduate school you have selected, how will you explore your academic and intellectual interests at the University of Pennsylvania?” Cornell has a 650-word SOP. University of Michigan’s is 550 words and University of Wisconsin, Madison, is 650.