… Why after changing the chord suffix font in document options nothing change in the the newer created items? … What is the advantage of having that setting in document options? When it works? …

The way Finale works, is by design, and what users have requested.

And it does work, although perhaps not in the way you expected.

In your first question - “Why after changing the chord suffix font in document options nothing change in the the newer created items?” - the important word is “created”.

It does not matter whether the suffixes are older created items or newer created items.

Try this:

In the Document Options, change the Chord Suffix Font.

Then - after the font change - create a new suffix.

The new suffix will have the new font.

Already existing suffixes are not affected.

Finale shows an asterisk * to tell you that already existing items are not affected.

In the Document Options, change the Chord Suffix Font.

Then - after the font change - create a new suffix.

The new suffix will have the new font.

Already existing suffixes are not affected.

I finally understood the working mechanism of this design system.
Thanks Peter for clarifying how it works, now I understand that my mistake was to imagine that the elements created could be the new chords inserted in the score, while Finale refers to the new chord suffixes created in the library.
now everything is clearer, thanks.
I was fooled by the idea of thinking that the "fonts" section in the document settings could act immediately, with the same reasoning applied by classic word processing and layout software.
Thanks for the very helpful answer