Functional drafting practice is one that uses the least amount of information needed to convey all the information necessary to manufacture part weldments and assemblies.  Functional design practice would use the least amount of time and file space to ensure a sound design.  With the onset of computer aided drafting, it has become easy to lose sight of functional drafting and functional design.  An example of a common violation of functional drafting and design is threaded fasteners that have the thread feature, the thread profile cut on a spiral helix.  Adding the thread feature increases the file size for each fastener by, roughly, 5 times or 500kb per 1/4-20X1″ fully threaded hex head screw.  If you have an assembly with 1000 fasteners, you have added 1/2GB of memory to the assembly for this non-essential feature.  Some assemblies can get so big, they becomes impossible, or impractical, to load and work on the entire assembly.  When assemblies become very large, the amount of data contained in the non-essential information can make it hard to rotate, view and analyze these assemblies.