The distinction between the name and the type is that the type, the value
of XTENSION, describes the structure of the extension, while
the value of EXTNAME provides a name for the extension but says
nothing about its structure. For example, the header for an ASCII table
containing information about Seyfert galaxies could have
XTENSION = 'TABLE ' and EXTNAME = 'SEYFERTS'.
Because the term ``type'' refers to the structure of an extension as given by the value of XTENSION, user definition of a EXTTYPE keyword is not recommended, though not strictly forbidden by the FITS rules.
As a sample application of hierarchical structure, consider an observing session in which several objects are observed. In addition to the actual observations, other information, say, about the observing system configuration or physical state, might be present. Some information might apply to the entire observing session, while other information would apply only to the observations of an individual object. In this case, an extension at EXTLEVEL = 1 could contain the information applying to the entire observing session as part of the header, while some information about the observations of individual objects could appear in the associated data in a master table. Each row in the master table would contain information about the observations of one object. One entry in each row would be the name of a table containing observations of that object. The observations would appear in separate table extensions with EXTLEVEL = 2 and EXTNAME equal to the name given in the master table. This concept has yet to be implemented in data for archive or distribution, and any software to support it is still in the developmental stage.
The grouping proposal, discussed in section 5.3, describes an alternative method of creating hierarchies.