Add 10 and you get the secondary parts like 31 for exterior windows, 32 for interior doors and so on. For example, exterior walls begins with 21, interior walls with 22, floors with 23, and so on. The best part of this classification, for me, is the use of two digits to identify an architecture element.
I personally use a Belgian classification system (or more exactly a part of) name BB/SfB developped by the Régie des bâtiments, a public service that manage most of the Belgian federal real estate. This way, you can easily select and even filter components from the Outliner dialog box. It is very interesting to give your components meaningful names. 6 Use a consistent naming convention for your components
