HDS files are known as container files and by default have the extension `.sdf'. HDS files contain data objects which will often be referred to simply as objects. An object is an entity which contains data or other objects. This is the basis of the hierarchical nature of HDS and is analogous to the concepts of file and directory - a directory can contain files and directories which can themselves contain files and directories and so on (see Fig ).
An HDS object possesses a number of attributes, each of which is
described in more detail below: