Tier1 is something that we run in CI - Linux x86_64, osx, Windows
Tier 2 - something that receives less testing, but is expected to work. Aarch64 on osx, Linux, Windows.
Tier 3 is something that we don't run in CI and may have regressions. I.e. riscv64, s390x, ppco, EOL or unreleased python versions.