primary.xml.gz 'file' questions

Hi -

I've been digging around in createrepo and I've noticed that
createrepo generates a primary.xml.gz file with several "files" tags.
The files and directories that end up in primary.xml.gz seem to be a
subset of the files and directories found in filelists.xml.gz.

For example, from:
ftp://rpmfind.net/linux/opensuse/factory/repo/oss/suse/x86_64/strace-4.8-4.3.x86_64.rpm

I get a primary.xml.gz with this:

      <file>/usr/bin/strace-graph</file>
      <file>/usr/bin/strace</file>
      <file>/usr/bin/strace-log-merge</file>

Where as filelists.xml.gz contains the files listed above and more.

Is there a reason why certain files and directories appear in both
xml.gz's and some just appear in filelists.xml.gz?

Is there a special set of RPM attributes (from
http://rpm.org/api/4.11.1/rpmfi_8h.html possibly?) that I could
examine to determine whether a file/directory should appear in
primary.xml.gz ?

Thanks,
Joe