ESCD or Extended System Configuration Data is a part of nonvolatile BIOS memory (usually in flash memory) on the motherboard of a personal computer, where information about ISA PnP devices is stored. It's used by the BIOS to allocate resources for devices like expansion cards.
The BIOS also updates the ESCD each time the hardware configuration changes, after deciding how to re-allocate resources like IRQ and memory mapping ranges. After the ESCD has been updated, the decision need not be made again, which thereafter results in faster startup without conflicts until the next hardware configuration change.
An ISA Configuration Utility (ICU) can be used to update the ESCD via PnP BIOS interfaces. The Microsoft Windows operating system Device Manager is an instance of such a utility (and more).