Asus RS500A-X6/PS4 - Manual (Page 60)

4-12 Chapter 4: Motherboard information
8. BMC header (BMC_FW1)
The BMC connector on the motherboard supports an ASUS ® Server
Management Board 4 Series (ASMB4).
9. Power Supply SMBus connector (5-pin PSUSMB1)
This connector allows you to connect SMBus (System Management Bus) to
the power supply unit to read PSU information. Devices communicate with an
SMBus host and/or other SMBus devices using the SMBus interface.
This connector functions only when you install the ASUS ASMB4.