Also known as perms or system bets, permutations are multiple bets where you place money on few events. They are quite similar to the accumulator bets (combo bets) with the only difference that in perms you do not have to predict all the outcomes of picked events correctly to get profits. All it takes is for one of the outcomes of the selected events to be exactly as you have predicted for the bet to be successful, for example, 2/3 or 3/4 bets. 

Read this article if you want to know more about permutations.

Among most popular full cover permutations (system bets) are Trixie, Patent, Yankee, Lucky 15, Canadian, Lucky 31, Heinz, Lucky 63, Super Heinz and the Goliath.