Considering this excerpt from help: "Select Always to replace both the screen
font and the printer font, even if the original font is installed on your
system. Select Screen to replace the screen font only."

My interpretation is that if there is a font substitution such that "Helvetica"
maps to "Luxi Sans" (or "Bitstream Vera Sans") and only "Screen" box is checked,
then Helvetica is substituted on screen only, including helpcontent. Right?

However, opening helpcontent yields that the <HeadN> text is not anti-aliased
(helvetica is not anti-alias capable under Linux with default fonts). i.e. I
conclude font substitution was not taken into account.

If, I were to check the "Always" box, I get correct font substitution (anti
aliased text this time). I believe font subst logics is reversed in
ImplFontCache::Get().