NEC Express 5800/320Lc - Manual (Page 318)

System Upgrade 8-17
The DIMMs with the same number are linked among the groups. When a DIMM is added to
one group, another identical DIMM should be installed to the socket with the same number in
another group. This rule is applied to the case of removal.
The linked DIMMs should be of the same product with same performance.
DIMMs should be installed in sockets, starting from the lowest socket number.