how to understand pkgconfig Requires: fields ?

[Drew Parsons]

I'd like to ask someone to clarifies the Requires fields in the
pkgconfig .pc files.
Allow me to use lib/XprintUtil as an example.  Its pc file (see
http://webcvs.freedesktop.org/xorg/lib/XprintUtil/xprintutil.pc.in?view=markup) has:

Requires: x11 xt printproto
Requires.private: x11 xp xt

What is the difference between Requires: and Requires.private:?

xp is the odd one out, needed only in .private. At which point does it need to be installed?

Thanks,

Drew