1. There is no "Feta" font. It's called "Emmentaler", but it consists
of
two sub-sets of glyphs: "Feta" (the classical notation glyphs) and
"Parmesan" (the ancient notation glyphs).
2. Since Emmentaler comes in 8 different optical sizes, the commands
to
load them only require the root name "emmentaler" to load them all.
3. The fonts are located, not in the system's font directory(s), but
in the
program's own font folder, typically
"[LilyPondDir]/usr/share/lilypond/current/fonts/".
4. To use Emmentaler (or whichever is the main font, if using an
alternative) for a grob that supports the font-interface, it can be
set via
the 'font-family property, but the value isn't 'emmentaler, it's 'feta
(sigh). It's a shorter name, which is nice, but it isn't _technically_
accurate.
5. In a \markup, the fonts can be accessed directly with the real font
name, like "emmentaler-23".
6. In the NR, section A.8 is entitled "The Feta font", but the
opening line
starts out with "The following symbols are available in the Emmentaler
font..."
In other words, there's some inconsistency floating around.