The perps on Reya DEX settle in rUSD. What does that mean?

To settle in rUSD means that all prices and PnL payments are denominated in rUSD. So, for example, if you follow the price of ETH on the Reya dApp and see it price go up from 3500rUSD to 3550rUSD, that means shorts pay longs 50rUSD. Funding payments and fees are also paid entirely in rUSD.

A direct consequence of this is that your margin requirements are also denominated in rUSD. Margin requirements, after all, are funds locked by the Network to ensure you have enough funds to pay for possible losses.