When comparing one interstellar empire with another, census officials
and financial lizards talk about things like populations, manufacturing
capacity, and gross domestic product.
However, most ordinary emperors just count how many inhabited systems they own.
In the early days of interstellar expansion, people counted inhabited worlds, meaning those where there was an established human presence.
measure soon gave rise to problems and ambiguities. Alongside the
primary world, do you count all the moons with permanent bases? Some of
them were nothing more than observation posts manned by a handful of
staff, clearly not worth counting, but others were larger than some
planetary colonies. What about those planets with no surface
populations, but with a thriving flock of orbiting bases? What about the
large mining populations spread thinly through an ore-rich asteroid
belt? Exactly what constitutes a "world" was becoming a nightmare of
arbitrary and meaningless definitions.
system is defined simply as a star system containing a self-supporting
permanent population. This measure makes no mention of how the
population is distributed or housed, so avoids many of the problems of
earlier definitions. The "self-supporting" qualification has some more
specific definitions attached, but essentially ensures that the system
has enough critical mass of population and resources to be significant
in political terms.