This bridge connects the Secondary PCI bus to the Tertiary PCI bus on the ISE board. It is a standard PCI-to-PCI bridge. Software on the ISE board typically configures this device to make all the present daughter cards accessible.
The Tertiary PCI bus has potentially more bus masters then the DEC bridge can arbitrate, so on the ISE board it has been relieved of arbitration duties by disabling its arbitor. The Image Memory Controller takes over the Tertiary PCI bus arbitration.