How about this:

Other units are "regional," in that they are not historically tied to our existing factions. They are available to people who conquer their regions, based on their historical or hypothesized (where they never interacted in history) relationships, determined through a number of factors, and again the instilled government type.As you can see, unit recruitment is quite complex.

emphasis on hypothesized (where they never interacted in history) relationships. or in other words a POD and its subsequent ATL.