We’re in that point in our education where we don’t get assigned seats by the teacher. Instead we choose our own seats, but the seats we choose are implicitly our assigned seats. Those implicitly assigned seats don’t have our names, but the teacher, your friends, and your other peers know you sit there.
When someone comes to class a little earlier than I do, and decides to sit in my seat, I get a little ticked off. Where am I going to sit now? I respect everyone else’s assigned seat and my assigned seat was perfect for me because I was either by my friends, I get a good view of the board, or I could hear the teacher much better. So again, where am I going to sit now?
It’s a rule that’s not actually a rule and it’s an assigned seat that wasn’t actually assigned. It’s something that we all just know, and should respect.