wpkg --force-distribution ...
The --force-distribution option can be used to allow installation of a package with a distribution that is different from the distribution of the target system. By default, though, a target system is distribution agnostic and therefore this option is not at all necessary.
When you create a target with --create-admindir command you can specify a Distribution field to lock all the packages to that specific distribution. Trying to install packages from any other distribution fails with an error.
Say you installed a target with the Distribution field set to stable. Then only packages that were built for the stable Distribution are accepted by default. Other packages require the use of the --force-distribution to allow their installation on that target.