Element Classes
Elements are objects contained by an object. For example, a folder can contain files and other folders. The element classes listed in an object class definition show what kinds of elements objects of that class can contain. An object can contain many elements or none, and the number of elements of a particular class may change over time.A file can be an element of other Finder objects in which files can be stored, such as folders, disks, or the desktop; so File is listed as one of their element classes. The definition for object class File that begins on page 60 indicates that a file can't contain any other elements.