How are i386 packages selected for x86_64 repos?

[Bruno Wolff III]

I am playing with making custom repos combining the latest rawhide with the
latest koji updates. (I have been pulling kernel and ati updates from koji
pretty regularly anyway and things are getting a bit dull with the beta 
freeze and i figure getting koji packages that haven't been pushed yet
will liven things up.) For my i386 repo I use all of the f10 (and inheritted
rpms with older tags) rpms that are i386, i686 or noarch arches.
I was thinking of trying the same thing with x86_64 but I don't think I want
to pull in all of the i386 packages and was wondering how the list of i386
packages was selected for normal x86_64 repos?