The NDF file format is based upon the Hierarchical Data System HDS (SUN/92) and NDF data structures are stored in HDS container files (which by convention have a file type of .sdf). However, this does not necessarily mean that all applications which can read HDS files can also handle data stored in NDF format.
To understand why, you must appreciate that HDS provides only a rather low-level set of facilities for storing and handling astronomical data. These include the ability to store primitive data objects (such as arrays of numbers, character strings, etc.) in a convenient and self-describing way within container files. However, the most important aspect of HDS is its ability to group these primitive objects together to build larger, more complex structures. In this respect, HDS can be regarded as a construction kit which other higher-level software can use to build even more sophisticated data formats.
The NDF is a higher-level data format which has been built in this way out of the more primitive facilities provided by HDS. Thus, in HDS terms, an NDF is a data structure constructed according to a particular set of conventions to facilitate the storage of typical astronomical data (such as spectra, images, or similar objects of higher dimensionality).
While HDS can be used to access such structures, it does not contain any of the interpretive knowledge needed to assign astronomical meanings to the various components of an NDF, whose details can become quite complicated. In practice it is therefore cumbersome to process NDF data structures using HDS directly. Instead, the NDF access routines described in this document are provided. These routines ``know'' about how NDF data structures are build within HDS, so they can hide these details from the writers of astronomical applications. This results in a subroutine library which deals in higher-level concepts more closely related to the work which typical astronomical applications need to perform, and which emphasises the data concepts which an NDF is designed to represent, rather than the details of its implementation.