The syntax is a bit awkward, but it is the same as for a package name
and introducing another syntax wouldn't really help usability, so with
apt install ./foo.deb/experimental you will get the dependencies of foo
satisfied by your default release, but if this wouldn't satisfy the
version requirements the candidate for this dependency is switched to
the version from the experimental release. The same applies for apt
build-dep ./foo.dsc/stable-backports which was the initial request.