The PCI Local Bus 2.1 standard specifies that every device has a 256 byte configuration space, the first 64 bytes arranged and understood the same for every kind of device. The behavior of configuratiion spaces for specific devices can best be understood in terms of the general behavior of the PCI specification.
The devices in the ISE board are arranged on two PCI busses--the secondary and the tertiary bus. The bus ids are assigned by the host BIOS initialization, but the device ids are fixed by the hardware construction.
Table 13: ISE Device ID Assignment