From the package review guidelines:
MUST: If the package does not successfully compile, build or work on an
architecture, then those architectures should be listed in the spec in
ExcludeArch. Each architecture listed in ExcludeArch needs to have a bug filed
in bugzilla, describing the reason that the package does not compile/build/work
on that architecture. The bug number should then be placed in a comment, next to
the corresponding ExcludeArch line. New packages will not have bugzilla entries
during the review process, so they should put this description in the comment
until the package is approved, then file the bugzilla entry, and replace the
long explanation with the bug number.
This Bug is used as a tracker for such issues. If you added a bug for a package
that uses ExcludeArch, mark it as blocking this bug.