Noninteractive is the word! Thank you.
This is a technical term covered by debconf(7) :)
The idea is covered here:
With a non-interactive frontend you can install all packages and configure them with one script.
If you want to patch configuration (files in ``/etc``) you can use either patch(1) or some VCS (like git).
But obviously people can call it declarative if the like to,
maybe a fancy word like that is what it takes to promote it.
There is a term "Configuration as code" (or "infrastructure as code"). The idea is to describe your configuration using a text file, store it in VCS and apply.
Lots of tools are available:
* Ansible
* Chef
* Puppet
* Terraform
Anyway how does that work in practice? Maybe it is even
described in the man page you refer to ...
debconf is covered in manpage. For best practices google for "configuration as code".
There is a good explanation, and even a book!