Fujitsu PRIMERGY RX2540 M4 Upgrade And Maintenance Manual page 337

Main memory
Dual-rank / quad-rank / octal-rank DIMMs
– Same letter represents identical DIMMs (capacity, rank).
– In Rank Sparing mode only one DIMM capacity per CPU is allowed.
– "x" must always be DIMM with more ranks than "z".
– If both DIMMs have same number of ranks, "x" represents DIMM with higher
capacity.
Upgrade and Maintenance Manual 337
RX2540 M4