Fujitsu PRIMERGY BX620 S5 Options Manual page 30

Memory modules
Mirrored Channel Mode (maximum security)
Only the memory modules installed in the two channels A and D (see
figure 13 on page 29) can be used up to 32 GB). Channels B and E are only
used to ensure failsafety; they contain the mirrored data of channels A and
D. Channels C and F are not used.
The two channels of each CPU that are in use (A&B or D&E) must be fitted
with identical modules.
This mode requires RDIMM memory modules.
Performance Mode (high memory configuration with highest performance)
All slots of a memory bank are used. The main memory can be configured
up to its full capacity with best
performance.
In Performance Mode, three identical memory modules must be used per
bank.
This mode requires RDIMM memory modules.
30 Options Guide
BX620 S5