A seat in class (most often a college class) that you normally sit in all the time; so much so that it practically belongs to you. More often than not, the majority (or even the entirety) of the class has one because they pretty much always sit in the same seating arrangement every class. However, if the seats are not officially assigned in class by the professor/teacher, someone else could sit in your unassigned assigned seat either without knowing that you normally sit there (perhaps they're new to class or just never noticed), or in rarer cases deliberately, causing you to have to find another seat with discomfort and awkwardness. You feel that you HAVE to sit there every time because that seat is yours.

When you eat dirt for the first time and leave your family to build a shack in the woods