A system bet becomes available when selecting at least three different events/matches to bet on. The number of possible system bets depends on the number of predicted outcomes. The main difference between a system bet and a combined bet is that with a system bet, you can win part of your bet even if you do not have all the outcomes correct.
If you prefer to place only singles within one bet slip, you may also choose 2 or more selections, click on "Single", and modify the stakes according to your preference.