DCPMM must be explicitly supported by the board, as it requires a UEFI configuration utility.
As I mentioned on your other post, these SAGE boards may or may not support Optane at all.
M, intuitively, stands for medium memory, not large memory. SKUs with an L suffix like the 6268CL support 4.5TB per socket. This value is the maximum sum of DCPMM + DRAM.