ASUS in conjunction with G.Skill and Zadak have designed a non-JEDEC UDIMM standard, called “Double Capacity DIMM” or DC DIMM. The whole idea of these modules is to max out the CPU memory controllers limit despite having only two memory slots on your motherboard. These will be especially useful in mini-ITX motherboards where you are limited to only two memory slots.
These could also come in handy on some LGA2066 boards that have only 4 memory slots. The first DDR4 DC modules could be 32 GB so you could max out the memory controller on Intel 8th and 9th gen processors with just two modules.
ASUS will be marketing this standard heavily with its upcoming Z390 motherboards. It will be interesting to see if just ASUS motherboards will support these modules or it will be all boards.