The stop-limit order feature in Poloniex is a very useful for trading, since we cannot really tell when a price action will happen we just have to put a stop into when our cryptos would sell or buy. The stop-limit's main use is to what the name implies, was to make you sell/buy to limit your loss or buy if the price touches a key price point.
It is like if the price hits 10 (the stop price) then if it goes to 9 and below you will get sold (limit) if you will limit set sell price below 10. So it must hit 10 before your sell price blow that will get sold off.
Here I bought Dogecoin at 48 Satoshis and I want to sell it at 80 satoshis price-point. So I would set the stop at 81 and the limit at 80 from where it will get sold out.
After clicking "submit" you will be prompted with a message telling you that if the highest bid falls to below the stop price your cryptos will get sold. This is the way I sell/buy my cryptos at Poloniex but the main reason for a stop-lit order is for you to stop your losses if a certain price is triggered.