The United Colonial Nations Senate is the legislative branch of the UCN. Its job is to govern the planet Earth, and to make rulings of the UCN's decisions regarding its colonial dominions. It is made of Earth only members, with the ISA colonies having no representational power within the UCN itself. The Senate members are the leaders of Earths' still independent nations and the CEOs of the most powerful and wealthy Earth-based corporations in humanities society.
The Senate functions as an oligarchy over the known galaxy, in which a comparatively small body of people hold all power within a government. The Senate is quite tyrannical in its rule, offering no forms of compromise or real negotiations to any disgruntled or unruly colony, typically solving any colonial disputes with either a threat of an extremely violent military attack if the colony does not accept everything the UCN dictates, or simply outright attacking the colony immediately.
The Senate takes no part in actual governance of the colonies, and typically does not bother aiding colonies that are not of vital tactical value, preferring to send the less equipped and less experienced ISA to die in their place. Overall, the UCN Senate only cares about making sure the flow of wealth and resources to Earth remains open, while also ensuring no colony reaches a technological level comparable to the UCN.