The minimum UDIMM/SODIMM interface configuration is single rank 64-bits while the maximum is dual rank 72-bits. For all single rank configurations there are two pinout options; either Optimum or Rank Expansion. For all dual rank configurations the only option is Optimum because it is already using the Rank Expansion pinout.
If future applications only use single rank devices, then leave the pinout at Optimum. If future applications require single or dual-rank devices, then Rank Expansion is required.
Generate the pinout for a 72-bit DQ width for future ECC support. Ensure you select the appropriate Command/Address Parity, DM/DBI, and Pinout Swapping options when generating the pinout. Generate the maximum future pinout as Dual-Rank. This ensures that the additional DDR4 output clock and control signals the second rank requires are included in the hardware design.