UC (Universal Credits?) is StarBreak's premium currency, used to buy cosmetics and quality-of-life features.
There are two ways to begin the process of buying UC. One way is to attempt to purchase something for which one does not have enough UC. The other way is to interact with the UC Kiosk in Eschaton Station.
Either way, the Buy UC interface will appear, allowing the user to spend from $5 to $50 USD at a time to obtain more UC to spend. Specifically, one can buy 50 UC for $5 (base), 110 UC for $10 (10% more value than base), 250 UC for $20 (20% more value than base), or 700 UC for $50 (40% more value than base).
UC is used to buy additional item storage chests (40 UC each), shell storage vats (95 UC each), and temporary hats (5 UC each) which are lost on shell death. The first two items are located in your home, while the last item can be obtained from the Hat Shop in Eschaton.