The term “unit” needs explanation. There are several interpretations about exactly what constitutes a unit. In a procedural programming language, a unit can be
A single procedure
A body of code that implements a single function
Source code that fits on one page
A body of code that represents work done in 4 to 40 hours (as in a work breakdown structure)
The smallest body of code that can be compiled and executed by itself
In an object-oriented programming language, there is general agreement that a class is a unit. However, methods of a class might be limited by any of the “definitions” of a unit for procedural code.
The bottom line is that “unit” is probably best defined by organizations implementing code. In my telephony ...