multiple architectures and subpackages

[Andrei Kazarov <Andrei Kazarov cern ch>]

Dear Gurus,

What is the scope of "BuildArch: "  instruction of a spec file in case 
if subpackages are defined? Does it have a scope of a subpackage, as I 
would assume or is has a 'global' scope?
My example is following:
(Note that I'm not _compiling_ my binaries with RPM, here I'm only 
_packaging_ pre-built binaries for different architectures, so they 
could be later installed elsewhere)

packageA.spec:
...
Name:           PackA
BuildArch:   $(myarch)

%package sub1
BuildArch: noarch
....

%package sub2
BuildArch: i386
...

One of subpackages should contain files for particular architecture  and 
the other one is containing files common for all architectures.

It results in that all subpackages are build for $(myarch) architecture, 
regardless what is defined in subpackages sections.

Is this expected or I'm missing something? I could create separate 
package for each architecture, but is there a way to do that in one 
.spec file?
The related question: can I create a .spec file for a number of 
different architectures (e.g for i383 and x_86_64), and then run 
rpmbuild -bb for each of them (on different machines/platforms)?
E.g.
...
%package sub3
BuildArch: x86_64

Thanks
           Andrei