If you're now skilled in the number rhyme and major system, it's time to upgrade to this system to further increase your powers. This system memorises numbers in sets of 4 which is why it's more efficient. Although a there is another system called the PAO system which encodes 6 numbers at a time, this system is used by 8 times world memory champion so I don't think it's necessary to learn the PAO system which also takes longer.