Hi
"Sans", "Serif" (and "Monospace", and a few others) are
synthetic
fonts composed from the fonts installed on system using the rules
defined on system and user-level fontconfig configuration.
They're not a simple alias, they can be a patchwork of fonts, and the
composition of the patchwork for the same rules depends on the actual
font files present on disk.
Application developers like to use them, because their presence is
guaranteed no matter what, so they don't have to guess what fonts the
user chose to install. As long as there is one or several sans-serif
scripts on-disk "Sans" will work.
The small subset of fontconfig rules we use is documented there
http://fedoraproject.org/wiki/SIGs/Fonts/Packaging/Fontconfig
for more info, read the existing fontconfig rules, upstream's
documentation (/usr/share/doc/fontconfig-2.5.0/fontconfig-user.txt,
man fonts-conf) and ask more precise questions there or on upstream's
mailing list. Fontconfig is very smart but its documentation is a tad
lacking.
--
Nicolas Mailhot