This will avoid having poudriere build you incompatible software, or fail to build it.
For example, if you currently try to build databases/pg_partman with DEFAULT_VERSIONS=pgsql=9.3, it will build using postgresql 9.4, and fail to package because of some run depends.
===> pg_partman-3.0.2 depends on file: /usr/local/lib/python2.7/site-packages/psycopg2/__init__.py - not found ===> Installing existing package /packages/All/py27-psycopg2-2.7.3.txz [pkg.absolight.net] Installing py27-psycopg2-2.7.3... [pkg.absolight.net] `-- Installing postgresql93-client-9.3.18_2... pkg-static: postgresql93-client-9.3.18_2 conflicts with postgresql94-client-9.4.13_2 (installs files into the same place). Problematic file: /usr/local/bin/clusterdb Failed to install the following 1 package(s): /packages/All/py27-psycopg2-2.7.3.txz