What are enhancement function points?
As was explained in a previous question, IFPUG identifies three types of counts: development project counts, enhancement project counts and application counts. The first and last are basically the same. A development project of 1,000 function will result in an application that has an application count of 1,000 function points. However, subjecting that application to an enhancement project of 500 function point may result in an application that is sized at anywhere from 500 to 1,500 function points. New, changed and deleted functionality is all added together when counting for enhancement projects. There are just as many enhancement function points involved in deleting an application as there were development function points involved in building it! All this means that enhancement function points must be considered on a different scale than development or application function points.
