The Universal Union (UU) is an intergalactic empire. If you're unfamiliar with HL2RP, and don't want to spoil it, I'd suggest you to not read this guide.
The UU is simply the inter-galactical space unit, or otherwise known as: Advisors.
In Half life 2 RP, this focuses on Earth, where they cannot live, so they're in Stasis pods, on life support.
The Advisors have got a tongue, they can use to plant it into your brain and collect knowledge you have.
For as far anyone is aware, who have played HL2RP before, including me, we've never seen ANY Advisor in a HL2RP themed server, mainly because the Model has got no Animations whatsoever, we hope that Kurozael or Conna is going to fix this in the future.
If you've played Half life 2, You know about Breen, and Breen has been named as Earth Administrator, but the fact is, he's got power over the earth, but the Advisors got the power over hím, and that's also seen in the end of Half life 2, where he makes a plea for a safer destination of the portal, which is denied by them. There are many referations to the Universal Union, they're mainly found on the Pods of the Advisors, which will say: UU-URB-12543.
URB is the only referation so far, which will probably mean URBAN UNIT, which means, he'll be stored in either the Citadel or somewhere else where he can live on Life Support.
For the rest, there's little known about the UU, all we know is they are the ones that invaded Earth and took it over.
Some see them as our saviour, some see them as our opressors..
It's all up to you..