What is the difference between the Base Size Unit and the Function Unit?

The Function Unit is a measure of software size, which represents the project scope or the amount of work to be done.  It makes sense to dscribe a book as a collection of chapters, or a building by the number of floors or rooms it contains.  The Function Unit allows us to describe a system as a group of functional or logical components, e.g., Function Points, features, reports, or interfaces.