The idea behind a parametric estimate is that you establish the measurement you are interested in against a unit that is meaningful to you. They are using the term module to represent a unit in generic terms. The unit could be square feet or square yards, produce release, gallons, or something like that. You are essentially creating a ratio in the most simplest terms so that you can compare equally one effort to the next. For example, burning 300 gallons of fuel to get from point A to B and comparing that to burning 500 gallons to get from point C to D is not meaningful. However, if you break it down to miles / gallon, then you can compare the two different driving efforts.
Here's an example:
Apartment A rents for $600. Apartment B rents for $800. You are trying to establish a rent price for Apartment C. They are all in the same geographic area. How do you do this?
In this industry, they establish the parametric of dollars / square foot.
If Apartment A is 1,000 sq. ft. and B is 1,300 sq. ft. and C is 2,000 sq. ft., what would you charge for apartment C?
Maybe $1,200 assuming all else is equal??