I got this explanation from Naoki Hirata
- Nightly are QA qualified smoke test builds; this is a manual change
over once the smoke tests are run and things pass. - Nightly-test are the staging for QA to smoke test the builds, or
further; I would not recommend using this channel at all. This
changes when QA needs a build to test, or use for testing FOTA or
other things. - Nightly-latest is an unqualified build that happens on a daily basis.