BTCMEX is a crypto derivatives exchange that expands trading possibilities. Users can open both Long and Short positions, which means there is a chance to make a profit not only when the BTC/USD price goes up but also when it goes down.
If a trader opens a Long position, he assumes the Bitcoin price will go up. In this case, a better price is the price higher than the position's Entry Price. And the price worse is the Price lower than the Entry Price.
But when a trader opens a Short position, he assumes that the Bitcoin price will go down. The better price for a Short position is the Price lower than the Entry price. And the price worse is any price higher than the position's Entry Price.