Don't know what they actually stand for but these letters denote the kit type as well as the motherboard type inside that kit:
RYH: motherboard type is NUC5i5RYB, kit has room for an extra 2.5" SATA drive
RYK: motherboard type is NUC5i5RYB, kit has no room for an extra 2.5" SATA drive, so is smaller than the K type.
Look at Next Unit of Computing - Wikipedia, the free encyclopedia for more info.
Thanks Paul. I see now they sell the boards separately to the kits. That would explain the unique model number for the board itself.
If you need further information about our Intel® NUC Kits please let me know. Intel® sells NUC boards without case for developers (custom build)
The 'B' in NUC5i5RYB stands for 'Board' or 'system board'. K and H denote the case size, but they have identical system boards.