The difference between a system's environment and your homeworld's environment. Habitability also determines the base population growth rate for a colony at that system. The colony and system that an empire starts the game with. Homeworld are prosperous but not burgeoning. The habitability of an empire's homeworld determines what types of systems that empire will tend to find preferable.
Income is equal to the population in production units.
War is a continuation of policy by other means. It is not merely a political act but a real political instrument. The efficiency with which resources can be extracted from a given system. The process by which resource are extracted from systems. Mining is performed by allocating colony spending for a given colony on mining.