Pesky minus sign.... :nonono: I didn't see it when posting although I've seen it mentioned before in relation to Rech.
My take on this (for now) is that 0 RUs is an economy in equilibrium. The world has just enough to satisfy its needs.
Worlds with + RUs are producing enough surplus to allow investment in big capital projects like fleets, atmospheric processing plants, those gold plated toilets.
Worlds with - RUs do not have any disposable economic power and there are shortfalls and scarcities in goods and services. Both government and market are struggling to satisfy the needs of society (in what is probably a loosing battle).
I don't take a negative RU to mean that the worlds economy is a net drain requiring outside aid to be pumped in, but rather that all the worlds economic efforts are sucked into maintaining society.
To attempt to answer Han's question; Rech's -378RUs doesn't mean it is infinitely weaker than another world, but it does give us a quick snapshot of the relative economic strength or weakness between worlds.
If we compared North Korea at a notional -500RUs and Germany at +500RUs we'd get an idea that Germany is efficient, well resourced, has good infrastructure and an adequate labor pool while North Korea is lacking in some or all of these. We can see that economically they are far apart but to understand why we have to look more closely at the rest of the UWP and background.
I don't think the T5 RU concept is faulty, but I do think that confusing it with T4's Pocket empires RUs that can actually be "spent" and turned into stuff is damaging.