I'm reading man hier, which says:
/etc
Contains configuration files which are local to the machine. Some larger software packages, like X11, can have their own subdirectories below /etc. Site-wide configuration files may be placed here or in /usr/etc. Nevertheless, programs should always look for these files in /etc and you may have links for these files to /usr/etc.
What does "site-wide" mean?
Is it synonymous with "system-wide", that is, for all users on the system?
The hier manpage is largely based on the Filesystem Hierarchy Standard, and searching the full text of that document leads to this text (emphasis added):
4.11. /usr/share : Architecture-independent data
[...]
This hierarchy is intended to be shareable among all architecture platforms of a given OS; thus, for example, a site with i386, Alpha, and PPC platforms might maintain a single /usr/share directory that is centrally-mounted. Note, however, that /usr/share is generally not intended to be shared by different OSes or by different releases of the same OS.
From that and the text quoted in the question, we can conclude that a site is some sort of logical grouping of systems; say, the PCs in a University lab. The entirety of /usr is supposedly shareable between hosts of the same platform; /usr/share and /usr/etc can be shared between hosts of differing platforms. The quote in the question says that configuration files shared across hosts in a site can be put in /etc or in /usr/etc. One example of such a configuration file is /etc/sudoers, which explicitly supports this by allowing rules to specify the host that they should apply to (see, e.g., Why does the sudoers file contain IP addresses? or The HOST variable in /etc/sudoers).
