4.4.1 Requirements for Conforming Extensions

  All extensions , whether or not further described in this standard, shall fulfill the following requirements to be in conformance with this FITS standard.

4.4.1.1 Identity

  Each extension  type  shall have a unique type name, specified in the  header according to the syntax codified in §5.4.1.2. To preclude conflict, extension type names must be registered   with the  IAUFWG. The FITS Support Office  shall maintain and provide a list of the registered extensions.

4.4.1.2 Size Specification

The total number of bits in the data of each extension shall be specified in the header for that extension, in the manner prescribed in §5.4.1.2.

4.4.1.3 Compatibility with Existing FITS Files

No extension  shall  be constructed that invalidates existing FITS files.