Keilim, Chapter 24, Mishnah 9. This mishnah belongs to the series of rulings that open with the words "there are three": a single type of utensil is presented in three different versions, and each version receives a different halachah in the laws of tum'ah. Here the subject is the container used by workers to haul manure and straw.
The opening words of the Mishnah are "Shalosh mashpelot hen": the hauling basket comes in three varieties. Each of the three was constructed of different material and put to a somewhat different use, and it is precisely those distinctions that decide which categories of tum'ah are able to grip it.
The manure basket
"Shel zevel, temei'ah midras." The basket in which manure is carried out to fertilize the fields is susceptible to tum'at midras. That category attaches itself to anything set aside for a person to sit or recline on, and this basket fits the description: the laborers, when they paused in their work, would flip it upside down and use it as a stool. Because it functions as a seat, midras applies to it.
The straw basket
"Shel teven, temei'ah tum'at meit." Regarding the basket in which straw is hauled, the Mishnah rules that the tum'ah of a corpse can take hold of it. It is a true receptacle, so it has the halachic standing of a keli, and no keli escapes tum'at meit. Midras, however, does not apply to it, for the simple reason that it is never sat or lain upon. Its weave is made of slender wooden strips, and anyone who lowered himself onto it would crush it; an item incapable of holding a person's weight was clearly never intended as a seat.
The camel netting
The third item is introduced with the words "vehapuchlatz shel gemalim, tehorah mikelum." This puchlatz is a mesh-like contraption plaited out of cords, employed when goods traveled on camelback: it was stretched and fastened over the bundles so that nothing would work loose and tumble off en route. Concerning it the Mishnah teaches that no variety of tum'ah reaches it at all. The tum'ah of a keli does not apply, since the gaps in the mesh are so large that such a netting holds nothing and is therefore no receptacle. Midras does not apply either: it consists of nothing but cords, and cords make an uncomfortable seat, so no one ever designated it for sitting.
Three baskets that serve the same general purpose, and three different halachot: one that carries the full stringency of midras, one that is a keli for tum'at meit alone, and one that stands outside the laws of tum'ah entirely. The deciding factors are how the utensil is built and how people actually use it.