Where are the manuals for debconf specific commands?

I'm currently creating a custom live-image installer. Anyways, I'm taking a look at all other packages in the binary stage and many (if not all) are using debconf to show progress bars and take inputs, etc. My problem is none of the [debconf](https://manpages.debian.org/stretch/debconf/index.html) manuals states about commands like

nor are there any helpful information in

/usr/share/debconf/confmodule

. Not even a google search with quotes ["debconf" "db_progress"] would lead me to any valid information. Where do I find them?