It is minted currency, but currency minted for stipends cannot be sold on the Lindex, and so represents a diminishment of future earnings.

A stable economy requires the currency pool to grow or shrink in direct proportion to the availability of goods/services within the economy. RL economies attempt to create that balance, but it is difficult to achieve, and leads to all sorts of problems. GDP and GNP are optimistic approximations.

The Lab manages to do it with elegant simplicity since all currency trading is - essentially - a foreign exchange activity. Stipends + the sales by Supply Linden neatly encapsulate the growth of goods/services availability within SL.