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 ![[*]](crossref.png)
).
An HDS object possesses a number of attributes, each of which is
described in more detail below:
HDS [1ex