This is an order type that works exactly like a limit sell order. The only difference being, instead of setting a limit price, the current buy price is automatically set as the limit price. This would help when you want the sell order to execute at the current buy price instead of the sell price. This would then practically reduce your buy/sell price difference to zero. Similar to a limit sell order, a market sell order would execute when the sell price reaches the buy price at the time of placing the order. The execution of the order is also not guaranteed in this order type and you can delete your market sell order anytime.
The limit order lets you choose the price you want to buy/sell a currency at. All you would need to do is set a price that you think the currency might reach some time in the future and place the limit buy order. The amount you set for the limit buy order would be locked in your INR wallet till the order gets executed, expired or deleted. When the market moves and if the buy price of the currency reaches the price that you set, it will get executed and you will receive your currency at the price that you had set it at. The execution of this type of order is not guaranteed as it solely depends on the market. You can however delete a limit buy order if you think the market is not moving in your favor and your locked balance would be available in your INR wallet.
As the name suggests this order gets executed instantly at the price shown to you on the preview page. This is the default order type and whenever you want to buy any cryptocurrency at the shown price and want it to be executed immediately, then this is the order type you would use.