From: Ol' Blighty
It says in the Galactopedia:
Actively migrate these races away from the colony to other colonies that will accept them. Passenger ships will pick up these races from the colony.
If you have enabled the empire policy setting "Use Penal Colonies to implement Enslave policy" then these unwanted races will be migrated to Penal colonies for enslavement there.
Enslaves these races, forcing them into hard labor. When a race is enslaved at a colony it’s growth rate will be zero.
For each race that you enslave, the corresponding empires of those races will take offense at your actions. The amount of offense is directly related to the population amount that you enslave.
Enslavement also provides a bonus to the colony's economy. The amount of the bonus depends on the proportion of the total population that is enslaved, up to a maximum of 50% if all of the population is enslaved.
If you have enabled the empire policy setting "Use Penal Colonies to implement Enslave policy" then some of your colonies will automatically be set as Penal Colonies (policy set to Enslave). Your other colonies with policy set to Resettle will migrate their unwanted races to the Penal colonies for enslavement.
What happens if I have not chosen "Use Penal Colonies to implement Enslave policy", but I have manually set one planet to "Enslave". Will it resettle the races to the slave world, or will it only enslave the current population and not send more slaves there, resettling them to other worlds instead.
Also, what happens if you have Resettle on all planets, will it resettle them to another Empire? Because this appears to be the default setting for Haakonish who I'm playing now. I'm just not sure if I need to create a single world to accept the other aliens or if it will handily ship them off to the empire next door.
"My body may be confined to this chair, but my mind is free to explore the universe" - Stephen Hawking