What does the CREATOR header keyword mean?

The CREATOR keyword records the tool that created the structure of the file, not the file itself. Tools that produce a modified version of the input file propagate this keyword from the input to the output.

Tools that create a new file type (e.g. a source list from an event file) will set the CREATOR in the new file. For example:

unix% dmkeypar sources.pi CREATOR echo+
dmextract - Version CIAO x.y

The CREATOR value records that this spectrum was created with the tool dmextract in CIAO x.y.