Say I'm interested in buying 300 USD worth of stock A and 700 USD worth of stock B and let's say, for the sake of argument, that it's possible to buy a fractional amount of those stocks (i.e. I could buy 100.4 stocks of A and 23.54 of B). I also want the trade to be executed immediately.
A market order is not the right order for this - it's parameter is the amount of stocks I would like to buy. Those stocks are then bought at prices starting from the bottom of the 'sell' section of the order book. So it doesn't answer my requirements, as although executed immediately, I'm not guaranteed to have exactly 300 USD worth of stock A and 700 USD worth of stock B when the orders are filled.
A limit order is also not suitable - the parameters are the amount of stock to buy and the price I'm willing to buy at. If I calculate those parameters carefully, it might still place my order in the order book and it won't be executed immediately.
What is the correct order type in this case? And if it doesn't exist, why?