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5.4.3 Additional Keywords Requirements

New keywords  may be devised in addition to those described in this standard, so long as they are consistent with the generalized rules for keywords and do not conflict with mandatory or reserved keywords. Restrictions

No keyword in the primary header  shall specify the  presence of a specific extension  in a FITS file; only the EXTEND keyword  described in § shall be used  to indicate the possible presence of extensions. No keyword in either the primary or extension header shall explicitly refer to the physical block size, other than the deprecated BLOCKED keyword  of §