Back in 2015, the NSE came up with the self-trade mechanism for traders who have both, buy and sell orders open for a particular scrip at the same price. You can check out this NSE document for more information. 

In the case of cover orders, the position will be left hanging with either no target or SL order, if you place a counter order matching the cover orders stop loss or target orders. 

Let’s understand this with an example. You place a Buy CO at market price (Last Traded Price is 200) with a Stop Loss of 199. Note that both these orders are placed simultaneously at the exchange. Before the execution of the first order, if the share price comes down to 199 then the Stop Loss will get prompted.