An NDF may contain any number of extensions which are distinguished by unique names. An extension name may consist of up to 15 characters. It must start with an alphabetic character, and may contain only alphanumeric characters (including underscore `_').
Extensions are normally associated with particular software packages which
may use a particular extension name to identify additional information
stored in an NDF in a form which only they understand.
For instance, a software package for analysing IRAS data might ``own'' the
extension name `IRAS' and use it to store additional information in NDF data
structures relating to the processing of IRAS data.
All the applications in that package would then be expected to recognise
this extension in all NDF structures and correctly process the information
it contains so that it remains valid throughout.
However, there is no requirement for other software to be aware of this
extension, other than to avoid using the same name.
In general, software which does not recognise an extension need do no more
than propagate it (i.e. copy it), if appropriate, to any output
data structure.
Normally, this will be performed automatically by the NDF_ system and is
considered in more detail in §.