Other than statements that seem to indicate some sort of player driven economy but with regulation by UEE made in mid-2013, they have not described the economic system in a lot of detail. I'm pretty sure they have not been concentrating on this, and haven't worked it out.
I'm not sure what you mean by physical form of UEE credit. If you mean is it convertible to real world currency, then I am of two minds. If they don't do that, then the economy can be based on any combination of bullshit they want. If they do that, then they must follow a model that doesn't a) encourage "gold farming", or b) violate any real world nation's tax laws.
One company proved a fully integrated game/real world economy works back about 10 or 15 years ago. The model (still operating) works and is capable of supporting players earning in excess of $100,000 US in some cases, but the company had to focus on the economic model, over the game engine in order to make it work. I don't think we want that just now.