One of the most important properties of HDS is that its data files are self describing. This means that each object carries with it information describing all its attributes (not just its value), and these attributes can be obtained by means of enquiry routines. An example will illustrate:
Here DAT_INDEX is used to get locators to objects about which (in principle) we know nothing. This is just like listing the files in a directory, except that the order in which the components are stored in an HDS structure is arbitrary so they won’t necessarily be accessed in alphabetical order.