dh_installdeb is a debhelper program that is responsible for installing files into the DEBIAN directories in package build directories with the correct permissions.
dh_installdeb automatically installs the following files from debian/ into the DEBIAN directory: package.postinst package.preinst package.postrm package.prerm package.shlibs package.conffiles
The postinst, preinst, postrm, and prerm are handled specially: If a corresponding file named debian/script.debhelper exists, the contents of that file are merged into the script as follows: If the script exists, then anywhere in it that ``#DEBHELPER# appears, the text of the .debhelper file is inserted. If the script does not exist, then a script is generated from the .debhelper file. The .debhelper files are created by other debhelper programs, such as dh_installmenu''(1), and are shell script fragments.