For example. If you long X number of shares at 10$, you can set a price over $10.01 to sell them at. You cannot set the limit order to "stop loss" of revenue (ie less than $10). If you set a stock that you hold long, to sell at a price below its current value, HSX considers the value met and sells it immediately.
The reverse is true for a stock you hold short. If you hold Y number of shares short, and its current value is, say, 20, you can set the limit order to any number below 20. The price will drop and when it achieves your limit, HSX will rid your port of the shares requested. You cannot set a stock that you hold short, to 'cover' at a value higher than its current price. HSX will consider the value met and cover it immediately.
I hope this makes sense and helps. It's a confusing option at first. Try it out with 1 share and practice a bit before using it extensively. This way, if you make a mistake, you don't lose a ton of commission.