I'm learning the Sugya of pas haba'ah b'kisnin. This seems like an extraordinary area of Halacha, because the beracha on this type of food is entirely subjective, based on what others would be koveah a seudah (what would count as a meal). Also, the Rosh says that young children and elderly people don't need to eat the same amount of food to qualify for meal as say, a middle aged man.
Does this kind of subjectivity exist in any other realm within Halacha?