It's a fraction, with different numbers and units on top and bottom, but with an overall value of exactly ' 1 ' with no unit.
When properly applied, it 'converts' a quantity with one unit of measure into an identical quantity with different units.
Example: to convert F feet to an identical quantity with the unit of 'inch', multiply (F feet) by the conversion factor
(12 inch) / (1 foot) .
This fraction is exactly equal to ' 1 ' with no unit, and the result of the multiplication is
(12 F) inch .
