ℹ️'Hostile' here means either the two empires are at War, or they're in Cold War with the besieger having the 'Righteous Fury' Religious automatic law, or they're in Cold War with the defender's system being within the besieger's cultural boundary, or the defender is an always-hostile minor faction like Pirates.
While a system is sieged, it can't refill its, send or receive any civilian ships, or produce any strategic or luxury resources for the empire. If a hostile fleet orbits an enemy system with no other enemy fleets distracting them, they automatically lay siege to the system, reducing the system's defensive manpower every turn.
