Stodgy and environmentally fanatic organisation enforcing Galactic Laws of Residence and Migration, determine which planets or whole areas of space will go fallow and for how long.
Also give out temporary permits to races for residence on specified worlds. Usually a planet's continents would be left fallow, with a settlers occupying only coasts or islands.
A Class C permit allows for a 300 million year residence. It allows humanity to purchase 3 existing colonies at Cynus shortly after contact.