An NDF extension is an HDS object, normally a structure, whose contents are entirely at the discretion of the extension's designer. Although some simple NDF_ routines are described below for identifying and accessing extensions, the contents of extensions are generally unknown and their interpretation lies outside the scope of the NDF_ system. This information must therefore normally be handled by means of HDS routines, so anyone planning to use NDF extensions in their software will need to be familiar with the concepts used by HDS (see SUN/92).
In addition, some general guidelines aimed at minimising the risk of poor design and eliminating name clashes between extensions are to be found in SGP/38. These should be consulted by all potential designers of NDF extensions. The following additional recommendation should perhaps also be added:
Do not design over-elaborate extensions.
The freedom allowed by HDS is a great temptation to do so, but the cost of
writing software to support the extension must always be kept in mind.
NDF [1ex