The SPD table is available in the SMBUS which is used by RAM, batteries and other hardware. The bus is used for configuration etc. This way a battery can be asked when it was manufactured or RAM can be asked what speed it runs at.
The SMBUS can support 127 devices which is more than any machine needs. Writing to the SMBUS is how the RAM flash memory can be changed.
SMBUS is part of the ACPI specification which is very complicated. The manuals are over 1000 pages and are generally far from complete.