Source RPM packages -- SRPMs -- have an architecture of "src". In other words, a source RPM is a source RPM, with no architecture associated with it. There's an assumption that the package is architecture-neutral in source form, and only become architecture-specific when built into a binary RPM (unless it builds into a "noarch" RPM, which is the case with scripts, fonts, graphics, and data files).
An SRPM contains source code (typically a tarball, and sometimes patch files) and a spec file which serves as manifest and build-recipe, plus metadata generated from the spec file when the SRPM is built -- including dependencies (which, unlike binary RPMs, are actually the build dependencies).
However, the build dependencies may vary by platform. If package foo is built against bar and baz, and baz exists on some architectures but not others, then the spec file may be written to build without baz (and the accompanying features that baz enables) on some architectures. The corresponding BuildRequires lines will also be made conditional on the architecture -- and this make total sense. However, querying an SRPM on a given platform may give incorrect build dependency information for that platform if the SRPM was built on another platform -- and only rebuilding the SRPM on the target arch will correct the rpm metadata (and possibly render it incorrect for other platforms). Thus, I've come to realize, SRPMs are not truly architecture-neutral -- and I'm not sure if all our tools take this into consideration.
Edit: I know that not all of our tools take this into consideration.
