Comment 1 for bug 1624681

Essential packages not installed will be installed automatically by APT, packages with the Important field will not be. Important also does not have the same ordering requirements as Essential. It's main use case is building configuration metapackages:

  https://juliank.wordpress.com/2012/01/24/managing-system-package-selections-using-custom-meta-packages/

But yes, this is not specified at all - it is an APT-specific feature. People do not like the field name, because it can be confused with Priority: important, so it has not been standardized and included in dpkg yet - also see https://wiki.debian.org/Teams/Dpkg/Spec/ImportantField and https://lists.debian.org/deity/2016/03/msg00025.html for further information.

Once this has a new name and is standardized we can actually produce good error messages.