It will be evident from the rest of this document that the standard components of an NDF (title, data, quality, etc.) have rather well-defined meanings and rules associated with them which govern their interpretation and processing. This is necessary in order to allow items of general-purpose software to be written (often by independent authors) in such a way that they can be used together co-operatively and without the different authors having interpreted the meanings of the various components in different ways.
In contrast, extensions are the means by which the information stored in an NDF may be extended to suit local or personal requirements. For instance, an extension may be used to hold information specific to a particular form of processing, a particular software package, a particular author, or a particular astronomical instrument. In this case, universal agreement about how to process the information is not necessary, so no restrictions are imposed on the information which may be stored in extensions. As a consequence, a clear distinction has to be maintained between the processing of standard NDF components and of extensions, since truly general-purpose software can only be written to process the standard components of an NDF, whose interpretation is universally defined. If an NDF contains extension information, then this can only be successfully processed by more specialised software.