universal_archs vs configure.universal_archs

[Ryan Schmidt]

On Oct 13, 2012, at 04:28, Joshua Root wrote:

> build_arch and configure.build_arch are not equivalent. The former
> reflects what is set in macports.conf and is not meant to be modified
> (and it's only a variable, not an option, to reflect this.) The latter
> is what will actually be used when generating the archflags passed to
> configure, can be changed by the portfile, and has a default value based
> on build_arch but possibly changed because of supported_archs.
> 
> configure.universal_archs and universal_archs are analogous to
> configure.build_arch and build_arch, respectively, and are used instead
> of them when the universal variant is set.

So, under what circumstances (if any) should a portfile use build_arch / universal_archs? Or should ports always use configure.build_arch / configure.universal_archs?