In compat 10 and earlier, this program was primarily for when upstream's build system does not properly install them as a part of its install step (or it does not have an install step). In compat 11 and later, it also supports the default searchdir plus --sourcedir like dh_install(1) and has the advantage that it respects the nodoc build profile (unlike dh_install(1)).
Even if you prefer to use dh_install(1) for installing the manpages, dh_installman can still be useful for converting the manpage encoding to UTF-8 and for converting .so links (as described below). However, that part happens automatically without any explicit configuration.
You tell dh_installman what man pages go in your packages, and it figures out where to install them based on the section field in their .TH or .Dt line. If you have a properly formatted .TH or .Dt line, your man page will be installed into the right directory, with the right name (this includes proper handling of pages with a subsection, like 3perl, which are placed in man3, and given an extension of .3perl). If your .TH or .Dt line is incorrect or missing, the program may guess wrong based on the file extension.
It also supports translated man pages, by looking for extensions like .ll.8 and .ll_LL.8, or by use of the --language switch.
If dh_installman seems to install a man page into the wrong section or with the wrong extension, this is because the man page has the wrong section listed in its .TH or .Dt line. Edit the man page and correct the section, and dh_installman will follow suit. See man(7) for details about the .TH section, and mdoc(7) for the .Dt section. If dh_installman seems to install a man page into a directory like /usr/share/man/pl/man1/, that is because your program has a name like foo.pl, and dh_installman assumes that means it is translated into Polish. Use --language=C to avoid this.
After the man page installation step, dh_installman will check to see if any of the man pages in the temporary directories of any of the packages it is acting on contain .so links. If so, it changes them to symlinks.
Also, dh_installman will use man to guess the character encoding of each manual page and convert it to UTF-8. If the guesswork fails for some reason, you can override it using an encoding declaration. See manconv(1) for details.
From debhelper compatibility level 11 on, dh_install will fall back to looking in debian/tmp for files, if it does not find them in the current directory (or wherever you've told it to look using --sourcedir).
Note that this is not the same as the --sourcedirectory option used by the dh_auto_* commands. You rarely need to use this option, since dh_installman automatically looks for files in debian/tmp in debhelper compatibility level 11 and above.
doc/man/foo.1 # Translations doc/man/foo.da.1 doc/man/foo.de.1 doc/man/foo.fr.1 # NB: The following line is considered a polish translation # of "foo.1" (and not a manpage written in perl called "foo.pl") doc/man/foo.pl.1 # ...