In general, for all high-level I/O functions, if the option parameter is 0, whatever HDUs are involved are either written to a newly created file or appended to the end of an existing file. In other words, no existing data is destroyed. However, if the option parameter is Overwrite, whatever HDUs are involved overwrite HDU number extver.
For putSingleGroup:
if option is 0, extver is ignored
if the file does NOT exist
the global header is written and
the single group is written
else if the file DOES exist
the single group is appended
endif
else if option is Overwrite
the file MUST exist
the single group overwrites HDU number extver
endif
For putSingleGroupHdr:
if option is 0
the file must NOT exist
the primary HDU is written
else if option is Overwrite
the file MUST exist
the primary HDU is overwritten
endif