Keilim, Chapter 24, Mishnah 4. This chapter runs through a long series of everyday objects, and each mishnah sorts one object into three categories according to the kind of tum'ah it can receive. Our mishnah applies that pattern to the wooden chest.
The mishnah opens: "Shalosh teivot hen" - there are three kinds of chests, and each one stands in a different halachic place.
"Teivah shepitcheha mitzidah, temei'ah midras" - when the entrance to the chest is set into its side, the chest can receive tum'at midras. Tum'at midras is the tum'ah that reaches an item designated for sitting or reclining when a zav or a zavah rests his weight upon it. Only something truly suitable for sitting enters that category, and this chest qualifies: because one reaches inside from the side, the top surface is free, and a person may settle himself on it while the chest goes on serving its purpose without any hindrance. Sitting and storing coexist here, so the chest is regarded as an article meant for sitting, and midras tum'ah applies to it.
"Mil'ma'alan temei'ah tum'at met" - when the chest is entered from above, it is capable of receiving corpse tum'ah, just as any utensil would be. It is a wooden vessel that holds things inside, and vessels of that sort become tamei in the ordinary manner. Tum'at midras, however, does not reach it, for a very practical reason: a person seated on such a chest covers the lid, and once the lid is covered the chest can no longer be accessed, which is precisely what the chest was built for. An article whose primary purpose is nullified the moment somebody sits down on it is not classified as an article for sitting, and therefore midras has no place here.
"Vehaba'ah bemidah tehorah mikelum" - a chest that comes in a measure, meaning a very large chest, is pure from every form of tum'ah. Utensils of such great size are not subject to tum'ah at all, because they are not objects that a person picks up and carries about. A vessel that is not movable is not treated as a keli for these halachot, and so this third chest stands outside the system entirely: not midras, not tum'at met, tahor from everything.
Three chests, then, and the difference between them is decided by something as ordinary as where the opening happens to be, and by how big the chest is. The halachah looks at how a person actually uses the object: a chest one can comfortably sit on without spoiling its purpose becomes a seat in the eyes of the halachah; a chest that must stay clear to be opened remains only a container; and a chest too large to be moved is no longer treated as a utensil at all.