All reports of dh-exec-subst-unknown-variable for the archive. The extended description of this tag is:
The package uses a variable in one of its debhelper config files, but the variable is not one known to dpkg-architecture.
It is recommended to use a known subset of variables. If the package needs more than that, and makes sure the variable is exported through the build one way or the other, then this tag can be safely ignored or overridden.
This tag has not been emitted in any package tested by Lintian.