It is written Rs, in standard cubic feet per stock-tank barrel. Above the bubble point a black oil cannot hold more gas than it already has, so Rs stays constant. Below the bubble point gas leaves solution and Rs falls. The values come from a PVT test.
The gas-oil ratio measured at the separator can be higher than Rs, because free gas from a gas cap or from gas that has already come out of solution can be flowing with the oil. Rs is a property of the liquid. The producing ratio is a property of the well.