From the manpage you linked for
Specify the minimum priority of question that will be displayed. dpkg-reconfigure
normally shows low priority questions no matter what your default priority is. See
debconf(7) for a list.
man 7 debconf:
Another nice feature of debconf is that the questions it asks you are prioritized. If you
don't want to be bothered about every little thing, you can set up debconf to only ask you
the most important questions. On the other hand, if you are a control freak, you can make
it show you all questions. Each question has a priority. In increasing order of
low Very trivial questions that have defaults that will work in the vast majority of
medium Normal questions that have reasonable defaults.
high Questions that don't have a reasonable default.
Questions that you really, really need to see (or else).
Only questions with a priority equal to or greater than the priority you choose will be
shown to you. You can set the priority value by reconfiguring debconf, or temporarily by
passing --priority= followed by the value to the dpkg-reconfigure(8) and dpkg-
preconfigure(8) commands, or by setting the DEBIAN_PRIORITY environment variable.
-plow will show all questions, irrespective of whatever default might have been set elsewhere. That might be want you want (that's often what I want, when I run