When you want to sell, (and this is what you do when you short!) you want to sell at as high a price as poss. So your Limit (Sell) is the lowest price at which you want to sell. If more, so much the better.
However there are occasions when you want to sell at a lower price when it occurs (act against your interests, in a sense). This is when you use the Stop order (sell) to distinguish it from the Limit order, (which acts in your favour). When for instance you believe that RBS now at 1510 is sure to retest 1470 but only if it goes below 1500. So you place your Stop order to sell at say 1497
The opposite for buying..
HIH