Refer to https://uefi.org/specifications

> ACPI defines many tables that provide the interface between an ACPI-compliant operating system, and system firmware. This includes Differentiated System Description Table (DSDT), Secondary System Description Table (SSDT), and Static Resource Affinity Table (SRAT), for example.

https://en.wikipedia.org/wiki/Advanced_Configuration_and_Pow...

As far as table contents, it's essentially an auxiliary DSDT (ACPI table defining variables, functions, interrupts, ...). An ACPI system has one DSDT and zero or more SSDTs.