A little adaptation of the ideal meaning is seen in the use pathaar-khan (the field). When we use khan with the earth as in dharaa-khan or prithibi-khan (both mean the earth), we are doing an idealization of a large spherical object to that of a large flat object. This is due to the perception we have that the earth is flat, a belief that ruled till about three centuries ago.
However, why does the same classifier apply to nadi-khan (the river) and to nao-khan (the boat)? In the case of the boat, it seems we look at the base of the boat and idealize it to something flat and somewhat broad. The same observation applies when we apply the classifier khan to baas-khan (the bus), gaari-khan (the bus or the car), relgaari-khan (the train).
In the case when we apply the classifier khan to a river, we are
idealizing the river to its top surface which happens to be
flat, a property of fluids. It is also broad.
This can be
used to explain the use of the khan classifier in
the case of a sea, as in xaagar-khan (the sea),
mahaxaagar-khan (the ocean).
The point of the discussion here is that although one can find an ideal usage for the classifier khan, the ideal type of object is not always found in the real world. In fact, it is very rare. As a result, we need to abstract out the relevant properties of the object and then idealize the geometry of the object to the ideal. Abstraction causes us to abstain from considering irrelevant properties of the object. For example, let us consider the use of the classifier khan with pathaar (field). This is not an ideal usage. If the field were completely devoid of any deformities (i.e., it is completely flat) and it did not have anything growing on it, only then it would be the ideal usage of the classifier. In general, most fields are not perfectly flat and have grasses and plants growing in them. In spite of the deformities and the presence of plants (e.g., paddy, beans, corn, etc.), pathaar-khan is felicitous in Assamese.
More complicated abstraction of relevant properties can be seen in the case of the use of the classifier khan with nadi (river) or xaagar (sea). A sea has waves and tides, so it is not completely flat. A sea has things of all kinds floating on its surface: ships, refuse, boats, people, etc. These are not taken into consideration when a native Assamese speaker uses the classifier khan with xaagar. Again, the existence of sea is not on the surface alone. A sea is actually a deep and uneven hole on the surface of the earth filled with water and other objects. However, the three-dimensionality of the sea is not taken into consideration when an Assamese speaker uses the classifier khan with xaagar. In other words, a native speaker of Assamese idealizes a sea to an object which is two-dimensional, completely flat and large.