Representative Fraction.  RF can be as either a fraction (1/10,000) or as a ratio (1:10,000) to define the linear relationship between the dimensions of the map and the area on the ground it covers, regardless of the unit of measure.  For example, OBM map sheets are produced with RFs of 1:10,000, meaning for every 1 cm on the map, there are 10,000 cm (or 100 m) on the ground.