Comment 8 for bug 217427

Source packages can now specify a Build-Indep-Architecture field containing an architecture spec on which to build the architecture-independent packages.

This isn't just a simple override of nominatedarchindep. Arch-indep will only ever be built on an architecture matching the indep spec, and a build will be created if necessary and possible even if Build-Indep-Architecture's value is mutually exclusive with Architecture.

One corner case when the field is omitted has changed: "all" now only forces a nominatedarchindep build if it is the sole term in the Architecture field. Previously an Architecture value of "doesnotexist all" would resolve to nominatedarchindep, but that must now be forced with a Build-Indep-Architecture.