https://wccftech.com/asrock-x299-motherboard-rdimm-memory-bios-support-2-tb-capacity/

ASRock has officially released the latest BIOS for their refreshed X299 motherboards, enabling the support for RDIMM memory with up to 256 GB capacities. Equipped on the X299 line from ASRock, these memory DIMMs can offer a total capacity of up to 2 TB which is a huge increase over existing 256 GB capacities which is supported by X299 boards from all other manufacturers.

It was already tested by ASRock's in-house overclocker, Nick Shih, that their X299 lineup would be receiving BIOS support for RDIMM memory which would let the motherboards support 256GB ECC memory in non-ECC mode. This would enable up to 2 TB of memory capacity support on ASRock's X299 motherboard which is something that no other manufacturer is currently offering.