The circular mil is a unit designed specifically to measure the area of a wire with a circular cross section. It has the symbol cmil.
An inch is 1000 mils. This makes the circular mil particularly useful for measuring cross sections of wire as there is no need to use the symbol π so the process is simplified.
A circular mil is a very small unit so not advised to use for large areas!
The square kilometre is a unit of area and is a multiple of the square metre; an SI derived unit. It has the symbol km².
A square kilometre is equal to the area of a square with edges 1km long. There are 1,000,000 square metres in a square kilometre.
Square kilometres are often used to describe expansive areas of land due to the large unit size.