In a system bet, 3 or more markets are combined. You can place the bet in the "Multiples" field at the bottom of your bet slip.
The biggest difference between a parlay and a system bet is that the second one can win, even if not all of the selections are correct, depending on the type of system that has been chosen.
For example, if your bet slip includes 4 selections and you choose the "Yankee" system, you can place the stake you want in the "Amount" field next to the system type. There, you can also see the odds, which will determine your potential winnings. Your bet will win if at least 2 of the 4 selections are correct, and your winnings will depend on the number of the picks that were correct.