Post by Sean Whitton
 This field should only be used when there are license or DFSG
requirements to retain the referenced source package.  It should not
be added solely as a way to locate packages that need to be rebuilt
against newer versions of their build dependencies.
Thanks, I think this is good -- would be good to hear from Nicholas too
I agree this is clear for people who already understand what it signifies,
but I don't think it's clear/accurate enough for a new contributor who
is struggling to understand Policy, because "retain the referenced
source package [singular]" seems to refer src:foo, if src:foo uses
Built-Using, and this isn't the case.
(3) The Built-Using field should exclusively be used to satisfy license or
DFSG requirements, where those requirements stipulate that the
specific versions of build-time dependencies must remain available in
the Debian archive.
With further consideration I think (2) should be cut and replaced,
because it undercuts the clarity of this paragraph's premise. Eg: Clear
(1||3) "should" premise, but if a tree falls in a forest and no one
notices it fall then you can do this other thing (2) without anyone
noticing. If a maintainer uses the field for (1), then it's a non-issue
if they're also privately using it as a heuristic for (2). Thus I don't
think we need to say anything about the (2), because it's confusing for
people who don't already understand the discussed concepts.
Rather than (2), I think it would be better to refer to the general case
of how to use foo.buildinfo (or tooling that leverages buildinfo, or
some other method) to identify packages that need to be rebuilt.
Sorry for the delay replying!