Efficiency of any organization, like fleet, spy network, army, industrial or trading complex, is determined by its HR points supply. They represent how well the entity is supplied by able humans.
HR points satisfaction is measured in percent, 50% being baseline efficiency. Up to 100% brings decent returns in efficiency, and anything extra costs a lot and is not very meaningful.
HR points are produced naturally by the population, every type production being proportional to the demand. As HR points get close to 50% satisfaction, its weight drops down to zero, so above 50% natural HR growth is only possible if all empire HR needs are satisifed.