Crop coefficients are properties of plants used in predicting evapotranspiration (ET).
Potential evapotranspiration (PET), is the evaporation and transpiration that potentially could occur if a field of the crop had an ideal unlimited water supply.
Even in agricultural crops, where ideal conditions are approximated as much as is practical, plants are not always growing (and therefore transpiring) at their theoretical potential.
Plants have growth stages and states of health induced by a variety of environmental conditions.
RET usually represents the PET of the reference crop's most active growth.
Kc then becomes a function or series of values specific to the crop of interest through its growing season.
These can be quite elaborate in the case of certain maize varieties, but tend to use a trapezoidal or leaf area index (LAI) curve for common crop or vegetation canopies.